Apple pay for in app purchase

In app purchases first bill the account balance, then proceeds to the first card listed as a payment method in your wallet. You can reorder those payment methods to change which card will be billed.

Apple Pay simply bills the card that you have set to use as the Default Card. Not sure if you are looking for Apple Pay to do something different.

Hi, Iris!

In-app purchases will be charged only to your Apple ID, and, therefore, your added payment methods. You can submit feedback directly to Apple here: Product Feedback - Apple.