Develop Reference

Google Play/App Store in-app purchase policies

I heard from someone that on App Store, everything you sell or anything the user buys, Apple must have their 30% on it. That would imply that even if you open a web page within the app and accept a donation, Apple must have its percentage. Otherwise you would have to redirect the user to make this donation outside of the app - for instance, calling the browser.
So, if it's true, I wanted to know if Google holds the same policies. I couldn't find this specific answer in the documentation, just info about in-app purchase (but I'm considering that scenario a purchase within the app, but not IN the app itself (sorry if it sounds confusing).
Taking Groupon as example. If you buy a coupon, is it an "in-app purchase"? I don't believe Google would take 30% on everything you buy there, so I just wanted to make sure it is according to their rules.
The app I'm working on will have both donation and coupon purchase from sponsors (Walgreens, etc). Is it ok if I do this through a WebView, or would I have to actually call the browser?
Sorry if my concept of "in-app purchase" is blurred.
Any information on this topic is much appreciated.
EDIT:
From my understanding, In-App Purchase (for both Google and Apple) is like an API that you use to process these payments.
What I find confusing is that, for instance, in the Apple's In-App Purchase Guidelines:
So, does it mean I cannot use Apple's In-App Purchase to sell my book (but could open my e-commerce site in the app and sell it), or I cannot sell my book at all inside my app?
I know it may sound like a dumb question, but it doesn't seem so clear to me.

The standard 30% transaction fee applies to in-app transactions on Google Play and goes to the distribution partner and operating fees.
According to the Google Play Developer Program Policies:
In-app purchases:
Developers offering virtual goods or currencies
within a game downloaded from Google Play must use Google Play's
in-app billing service as the method of payment.
Developers offering additional content, services or functionality within another category of app downloaded from Google Play must use Google Play's in-app billing service as the method of payment, except: >
where payment is primarily for physical goods or services (e.g., buying movie tickets, or buying a publication where the price also includes a hard copy subscription); or
where payment is for digital content or goods that may be consumed outside of the app itself (e.g., buying songs that can be played on other music players).
Reference: https://support.google.com/googleplay/android-developer/answer/112622
With In-App Purchase on iOS and OS X, you can offer your customers additional digital content, functionality, services and even subscriptions within your paid or free app.
For example, In-App Purchase will allow you to sell:
Digital books or photos
Additional game levels
Access to a turn-by-turn map service
Subscriptions to digital magazines or newsletters
Digital content hosted on Apple servers
There are four supported categories of In-App Purchase items that you
may sell: 1) Content, 2) Functionality, 3) Services, and 4)
Subscriptions. You must deliver your digital good or service within
your app. You may not use In-App Purchase to sell real-world goods and
services.
Reference: https://developer.apple.com/in-app-purchase/In-App-Purchase-Guidelines.pdf

I would have commented on another answer that is missing a bit of information, but as I lack the required reputation, I'll post this as a new answer.
It's 100% possible and allowed to sell physical goods or services in an app released on both iOS (via Apple Store) and Android (via Google Store), but the main difference between selling physical good or services and digital goods comes in the payment system to use. Both platform actually have 2 payments systems with, each, their own restriction and legal implementations.
In the case of Google Store (a.k.a. Google Play), it's as this:
Any digital purchases that adds anything into the app itself must be done through Google Play In-app Billing system. To put it simple, this is the typical in-app purchase you see in any game that charge onto the Credit Card registered by the user on the Google Play account.
Any Services and Physical Goods that are not set toward in-app usage have to use either your own version of a selling system (which has to be separated from Google Play In-App Billing system) or, if you want an more secure way, by using the Google Pay API.
The Google Pay API is really similar to the Google Play In-app Billing system as it allow any user to also use the credit card registered to their Google account (note that it's not Google Play, but a general Google Account), but the main difference is that the Google Pay API can also be used outside of an App (like on a website).
An example of stuff you must use the Google Pay API and NOT the Google Play In-App Billing system is to purchase physical goods (which requires a shipping address to be registered with the sell of goods) or services (like paying a rent of a room at an Hotel or the purchase of plane ticket, digital or not).
Here's the link toward the Google Pay API official website: https://developers.google.com/pay/api
For iOS and its Apple Store, the same equivalent exists, but the problem comes from the fact that both methods uses the same base system called Apple Pay.
For sells of goods and services, you need to address to the site "for merchants":
https://support.apple.com/HT204274
https://developer.apple.com/apple-pay/
For digital goods that are accessible directly through the app itself, you got to use the In-App Purchase system which is described here:
https://developer.apple.com/in-app-purchase/
There's one important thing you got to remember: Both stores might have some resistance to the idea of publishing an App as an App and as a Game. Both payment systems have their own rules and guidelines and, as such, if you are to put both a in-app purchase and an option to sell goods and/or services, both has to be visually separated and must NOT be mixed from one to another. For example, they might refuse to publish a game that has only physical goods to sell or an App in their Store app that is more of a game with in-app purchases and barely any instance of real-good purchases.
The cost of using either Google Play In-App purchase or Apple Pay In-App purchase is 30% of the sales.
On the other hands, Apple Pay for merchants (physical goods & services) have no additional cost from the credit cards issuers' fee which is usually around 3% to 6% based on the payment methods. For example, if the user is using Paypal to pay on Apple Pay, it's 2.9% + $0.30 USD per transaction. Direct Mastercards and Visa users might pay up to 6%. This is done and managed directly by Apple Pay.
Google Pay (for merchant) is also free and come with a fixed percentage rate of 2.9% per sales with credits cards and, in some countries, offers free transaction for direct-bank-transfers and debit cards.
In both cases, there's a limit per transaction. Google Pay has transaction limits, but their depends on the shops and user (shopper) data. I have seen the number $10,000 often as a limit per transaction.
Apple, technically, has limits or security based on the country where the purchase is placed. (All numbers are here: https://support.apple.com/HT207435 )

Publishing apps to google play

OK so in order to make a Google play account to distribute, I have to like make this Google wallet thing.. I want to know if it's allowed to do this if you are under 18. Also, for things like AdMob do you have to be over 18? Thanks!

you cant sign up for a adsense account(AdMob ) if you are under 18
read this

Taken directly from admob's site
If you are not yet 18, you may have a parent or guardian submit an application using their own Google Account. If their application is approved, all payments will be issued in this person’s name.
Google Wallet's ToS
By agreeing to these Terms of Service, you represent the following:
You are between 13 and 17 years of age and creating a Google Wallet account for the sole and limited purpose of redeeming Google Play Gift Card value for select items that are eligible for purchase by you on Google Play, subject to applicable laws and upon Google's discretion;
or
You are 18 years old or older; and
Capable of entering into a legally binding agreement.
So for admob you can use a parent's or guardian's assistance (so they will earn the money on their account), while on Google Wallet it seems that you can't earn money under 18, you can probably get a parent/guardian's assistance as well.
Keep in mind you don't need Google Wallet for free apps, I've uploaded many free android games but I'm not allowed to own Google Wallet where I live. You only need it for paid apps (or in-app purchase), you don't even need it for ads (like admob).

I have also wondered about this, but the simple answer seems like no unless you were to have your parents to do some parts for you.
This was taken from the Terms of Service just for a Google Play account:
"Age
Only users 13 years of age or older are permitted to use Google Play, and users under 18 must have their parent or legal guardian's permission to use Google Play."
"
More information can be found from the Google Play Terms of Service
That is only to use Google Play. On the other hand to become a developer, in order to distribute your application (Most likely a android app), you would have to pay a initial 25 dollar fee, and go on with additional steps, as shown here
In order to pay the initial fee, you would have to prove a valid credit card information which requires you to be the age of 18.
On the other hand, for AdMob you would have to read their own license and term of services just like you would for any third party application or service.
If you liked my answer, please mark this as the approved answer.
I will try to improve this answer as I gather more information.

Possible repulsion effect due to Google Play Services if the user doesn't have a Google+ account yet

I'm still in the process of developping a game and was excited when Google Play Services was announced during Google I/O and included it in my game a few days after. And it's great.
However, I fear that it may be a bad thing to use it since the user is requested to have a Google+ account to use Google Play Services and fear that the user will be repulsed if he hasn't a Google+ account yet and has to do all the process of creating one (a few steps, one being to type you first and last name, and your gender, and another sending you an email to welcome you) when launching my game for the first time.
Has anyone already published a game with Google Play Services and has an experience to share, be it good or bad? Did people react negatively to it in the reviews? Have the downloads decreased since its integration?
Since my game is fully multiplayer, I used the Android user's Google account to sign-in, so it made sense to make the use of Google Play Services automatic. For those who already published games with GPS, have you made it optional or mandatory?

The best approach is to make the sign-in optional, but as you described, this is very important to your users, so I would make sure this is very clear on the app's description!
I published the Minesweeper 3D with leaderboards and achievements, but they are optional. If the user doesn't sign-in he will see only his local scores.
I didn't notice any change on the number of installs/downloads, which is sad because we can't say anything about "google play games" on the app's description and then only apps they (Google) choose get attention from the public and millions of downloads.
But the number of sign-ins is extremely low, only 10% is pressing the sign-in button (clicks / total downloads).

does google charge for purchases in app?

I am building a free android app in which there should be a "store" where you can buy some item.
The plan is that user could pay for it using paypal or something. Now my question is, does playstore takes some percentage for such transaction? And if so, how much does it take?

The accepted answer is correct, but we should take into consideration another aspect about purchasing via smartphone/tablet. This recalls #JaranJhanjee's downvoted answer which is wrong but contains a little important truth
The 30% fee is charged by Google for in-app purchases according to their terms
Paid and Free Apps
App purchases: Developers charging for apps and downloads from Google Play must do so by using Google Play's payment system.
In-app purchases:
Developers offering virtual goods or currencies within a game downloaded from Google Play must use Google Play's in-app billing service as the method of payment.
Developers offering additional content, services or functionality within another category of app downloaded from Google Play must use Google Play's in-app billing service as the method of payment, except:
where payment is primarily for physical goods or services (e.g., buying movie tickets, or buying a publication where the price also includes a hard copy subscription); or
where payment is for digital content or goods that may be consumed outside of the app itself (e.g., buying songs that can be played on other music players).
Google always charges the 30% via its in-app billing API. But there are cases in which such an API should be avoided to not incur in an excessive charging
If navigator maps are usable across more devices (especially non-Android devices like iPhones o MS Phones)
If maps will be delivered packaged in an SD card at the customer's home address
In such cases, the seller is allowed to implement a different payment gateway like Google Wallet API or PayPal

Google charges 30% for every transaction. Details can be checked here.

Refer below link for google charges.
You receive 70% of the payment. The remaining 30% goes to the distribution partner and operating fees.
https://support.google.com/googleplay/android-developer/answer/112622?hl=en

For apps and in-app products offered through Google Play, the
transaction fee is equivalent to 30% of the price. You receive 70% of
the payment. The remaining 30% goes to the distribution partner and
operating fees.
As of January 1, 2018, the transaction fee for subscription products
decreases to 15% for any subscribers you retain after 12 paid months.
If a subscriber has been active as of this date, that time will be
counted. For example, if a subscriber has been active for 4 months,
the transaction fee will be reduced to 15% after 8 more paid months.
Reference

Confused about needing a Google Wallet Merchant account to enable in-app billing

I've pretty much only found out that "You must have a Google Wallet Merchant account to use Google Play In-app Billing." But I don't understand... What is it for? Do I actually need this? What does it do? How and where do I get one, and does it cost anything? The Google Wallet website seems to only have information for actual stores, not apps.
What I want is there to be a free, limited version of my app, but the user could pay a monthly subscription to get access to the full features.

I found out that what they mean by "Google Wallet merchant" is just "Google Checkout". "Google Wallet" has two parts, the app and online. The app is just for NFC purchases and nothing else, you cannot use any prepaid money on the Google Wallet app to buy anything on Google Play. The other part is the payment system that Google runs. This second part was formerly called Google Checkout, but a few months ago they merged Google Checkout and the Google Wallet system into one name to keep their money systems consistent.
TL;DR - you do not need Google Wallet to use in-app billing. You need Google Checkout, which is easier to register for and free.