How can I process payments in my app?

You have several options for processing payments in your app:

Only payments with card: It's only allowed to pay through card. The money will go straight to your account, depending on the processor you use..
Only payments at delivery: It's only allowed to pay at delivery. It can be through cash/transfer or any other method you want to use. Those payments are often considered to be "outside" of the app.
Payments with card and at delivery: You can receive payments both ways.
Just observation: Users cannot order through the app. Just for visualization of the catalog.

What credit card processors can i use?