Hello,
I am able to provide you functionality of payment gateway like today the Paytm ,phonepay and many more are rising and easy to implement
The Paytm Payments SDK for iOS and Android enables merchants to integrate payments into their mobile app.
User Placing the Order: The end user who wants to initiate a payment transaction.
Merchant Application: The Application of the merchant where the Paytm Wallet needs to be integrated. This may be an Android or iOS application.
Paytm SDK: The SDK provided by Paytm to make all the API calls to Payment Gateway and Merchant’s Web server (which is explained in the next point).
Merchant’s Web server: The merchant needs to setup this web server at their end. The logic to generate and verify CheckSum needs to be setup on this server. Checksum helps ensure the integrity of transactions i.e. it helps ensure that the transaction requests have not been intercepted and tampered with. Checksum utilities are provided by Paytm for different development platforms.
Payment Gateway: The Payment Gateway of Paytm. It will communicate with the Paytm SDK and Merchant’s Web server. In case, the payment is made via credit cards/debit cards/net banking, the Payment Gateway will interact with the Banks as well.
another is
Using Stripe vs. In-App Purchases
Google's developer terms require that purchases related to the app, such as premium features or credits, are made via their native In-app Billing API.
use Stripe for purchasing physical goods (such as groceries, clothing, and appliances) or for services (such as club memberships, hotel reservations, and tickets for events).
Stripe has created a Java library for Android, allowing you to easily submit payments from an Android app. With mobile library, we shoulder the burden of PCI compliance by eliminating the need to send card data directly to server. Instead, libraries send the card data directly to servers, where can convert them to tokens.
app will receive the token back, and can then send the token to an endpoint on your server, where it can be used to process a payment, establish recurring billing, or merely saved for later use.
thanks