Easiest is relative, but let's give it a shot.
Google remembers the Pay4App SDK more than it
should. That open source project carried on but outside a marketing website so to speak, will put something proper up since it's something people increasingly ask for and use.
Anyway we began a major rewrite of it sometime ago. Basically what we have is a extensible online payments framework that allows you to have a personal payment gateway in that funds comes straight to you. On the code side you have a payment method-agnostic interface and on the other framework side the ability to add and remove payment methods as need arises. So you can work with several payment methods. Can be say EcoCash using the line in your phone (or their API), PayPal, Zimswitch etc.
It's quite easy to wrap the above or any gateway in the other answers in a mobile app. If an Android app you can wrap the checkout in a WebView and set the return URLs and cancel URLs to be something like http://example.com
, the WebView allows you to capture redirects to certain URLs and perform actions such as read parameters and close the WebView (anything basically). You just have to see if the 3rd party gateways don't break UX/UI wise when wrapped in a WebView. But if you want to try out the developer preview of the project I mentioned prior, I tweet by the same username as here.
Hope that helps