I am developing a mobile app whose registration will be allowed only for Business entities (users will be verified and activated after each registration). In the app, Business entities will be able to purchase digital services that can be used within the app or linked website (no videos, cars or magazines). But the most important thing is that for each purchase they will have to receive a legal invoice (issued by my company) for the exact amount purchased. Am I forced to use in-app purchases or can I use an external payment system (perhaps without linking it directly within the app and sending a communication via email to the user)?