I'm not much an apple user, but when I got my ipad years ago I had a credit. There is $66 left - occasionally I see that amount next to my name. How do I use it? I bought a gift card but didn't have the option to use any of it.
It should automatically be used for your purchases (unless you are gifting items or an amount or are the organiser in family sharing). What happens when you try to buy something ?
