We are developing a Point of Sale app available on iOS and Android platforms. 

Our business model is such that, each user will have to pay us a customised cost based upon the number of his business outlets on half yearly or quarterly basis. Once the user will pay us that cost than he could use our apps on any number of devices, on any platform. 

I want to know if it's compulsory to process subscription charge for iOS App through Apple In App Purchases. Or we can process subscription offline and show user a alter message to renew subscription directly from our portal if a user's subscription expires.