Can I use a third party payment system if the goods are digital but not through the app?

My app is similar to a workout app where users use the app to manage their account (among other things) we are adding a feature for a personal trainer for a subscription fee, the only portal for users to interact with the system is through the app so we want to add all the payment related stuff inside the app

Our users are primarily in the US

I tried going through the terms of service but didn't get any clarity

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Can I use a third party payment system if the goods are digital but not through the app?
 
 
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