Family Sharing

You can't use your payment method when you are part of Family Sharing. Only the organizer's credit card can be used.

Buy/redeem iTunes Gift cards to the account. Charges will be deducted from the account balance before the organizer's credit card is charged.

App Store & iTunes Gift Cards by Email

Gift cards redeem

Gift Card - can’t redeem

Here's an article explaining how payment works when using Family Sharing:

Family purchases and payments - Apple Support

Except it has an error and is misleading. It omits to mention that an individual with account balance will have their account chanrged first before any remainder being charged to the organizer's account. That is clearly stated in: How App Store and iTunes Store purchases are billed - Apple Support