Apple is now documenting that a positive Apple account balance (done with iTunes Gift cards) will be charged first, so that is an option.
Where does it actually say that? It would go against one of the primary tenets Family Sharing works the way it does and is reined inside a tight relationship unit by the payment method system. The sections I quote below are specific as to payment methods. As is not unusual in Apple documentation the section you posted is a vague statement by saying "most", unless they are basing this on statistics from their sales department and actually sharing something like that with the public.
How apps, content, and subscriptions from Apple are billed - Apple Support
"Some purchases aren’t charged to your Apple ID balance
- If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method."
and:
"The primary payment method is at the top of the list in Settings on your iPhone," with a link to:
Change, add, or remove Apple ID payment methods - Apple Support which shows examples in the first image. The top part shows the person's Apple ID balance, below that they are all cards of some sort. At this point Apple's sometimes poorly worded and badly cross-referenced documents add confusion to interpretation since the top of the "list in Settings" is the Apple ID balance. However, the image also includes text, "The payment method below your Apple ID balance is used for purchases made by family members." I think "the list" really means "the list below your Apple ID balance".
I don't see where there is any mention of purchase sharing being linked to payment method used. Purchase sharing simply presents the option to let others share what you have bought, or not. "When you turn on purchase sharing, everyone in your family gets access to the apps, music, movies, TV shows, and books that family members buy from Apple."