How can I change my Payment Method in Family Sharing?

From: Add a family member to your shared subscriptions - Apple Support

"If you're the family organizer and you don't want to pay for family members' purchases and subscriptions, you can turn off Purchase Sharing [ How to share apps and purchases with your family - Apple Support ]. When Purchase Sharing is turned off, everyone must use their own payment method. When Purchase Sharing is turned off, family members can share subscriptions, but they can't share individual purchases like apps or books."

Remember that nobody replying to you here works for Apple so your platform decisions don't really affect us or influence our responses.

I can't think of any reason why leaving a family group would change your ability to use account balance. It is always charged first even if you are in or out of a family group, unless you are buying something for which balance cannot be used.

Redeem gift cards into your individual account, or ask her to turn off Purchase Sharing.

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