Any of those help OpenCart is implemented in some kind of MVC pattern. In order to implement a new payment module you could take a look at any of the already existing (bundled) payment modules. Pick any of them or more of them that could suit you the most and start by copying and editing it. The payment module files are located at these folders:
I wish this help you One common reason to get this error is if you have multiple Stripe accounts, and the API key you are using is not for the same account that was approved to accept Alipay payments. If your API key is for the correct account, then you'll need to write into support to get this sorted.
I hope this helps you . You can't use other third party payment methods for the purchase of digital goods/content with your app which is going to be consuming with in the app. So for that apple recommends the use of in-app purchase.