According to an updated support document, customers in assisted countries can now set up Apple Pay as a payment method for purchases made in iTunes and App Stores, and cover their Apple Music subscriptions and iCloud upgrades with Apple Pay.  This also applies to the purchase of Apple Books.
MacRumors explains that people may want to use Apple Pay for purchases related to their Apple ID because the mobile payment system is safer than other payment methods.
In addition to that, don't forget that Apple Pay allows you to choose from multiple credit or debit cards in flight, when you do these stores, and that is something you just couldn't do until you changed your Apple. ID payment method manually.  This feature is currently supported in eight countries:
- Hong Kong
- United States  You Need just link a credit or debit card with your Apple ID account:
1) Open the app Settings on your device.
2) Select iTunes and App Store from the main list.
3 ) Touch your Apple ID email at the top and then select ] See Apple ID from the menu.
4) Select Manage Payments .
5) Touch Add payment method .
6) Select a card listed under Found in the Wallet heading.
Of course, you must first add a credit or debit card that you want to use with store purchases in the Wallet app. This is a server-side change so no update is needed.
US users can also link an Apple Pay Cash card to their Apple ID account.