The inventory (title, SKU) is not managed by Piano, it needs to be set up in the respective app store. The paywall can be shown via an HTML template that’s fully customizable. Then a button would need to invoke the in-app purchase.
After the External service term is created, the verification works via receipts.
Piano takes receipts from Apple/Google stores and then tracks the user’s entitlements.
Apple/Google controls pricing. It’s possible to call the Piano API directly to submit the receipt but most of our clients use middle-ware to do this.
For synchronizing in-app purchases back to Piano you should refer to the documentation here.
If you should charge a slightly higher price for in-app purchases to make up for the revenue share of Apple or Google, we have seen some of our biggest clients not doing this. But some clients do and some get them to purchase on their own properties.