Use iTunes credit to purchase items
iTunes Store credit can be used only for purchasing content sold on the App Store/iTunes Store, or for upgrading iCloud storage. Official instructions can be found here:


Redeem App Store &amp; iTunes Gift Cards, Apple Music Gift Cards, and content codes


Selling of physical goods is not permitted on the App Store as per the App Store Review Guidelines. Refer specifically to the Business section of the document:


  3.1.5 (a) Goods and Services Outside of the App: If your app enables people to purchase goods or services that will be consumed outside of the app, you must use purchase methods other than in-app purchase to collect those payments, such as Apple Pay or traditional credit card entry.