Personal account balance always is tried first.
From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support
"Which payment method is charged?
When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:
1. Apple attempts to use your Apple ID balance to cover the full amount.
2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.
3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again.
If this involves a subscription and you do not have personal account balance, you can re-prioritize your payment methods (cards) so that the one you desire to be used appears at the top of the list when the payment comes due.
The alternative (which I think is what was being suggested in another post) would be to create a separate ID and use that to make the purchase you want for one account. Then share those purchases with Family Sharing. In that case you would need to carefully arrange the payment methods and the amounts you have in personal balance depending upon which account you wanted to have as the Organizer because Family Sharing has its own set of criteria for how purchases are paid for.
From: How to share purchases with your family - How to share purchases with your family - Apple Support
"When you turn on purchase sharing, everyone in your family gets access to apps, music, movies, TV shows, and books that family members buy. The family organizer is billed for family members' purchases."
Ideally the second sentence would also mention that others can still pay for their own items, they just have to use personal account balance to do so (it is mentioned if you go to the tiny footnote near the bottom of the page and follow the link there). Turning on purchase sharing simply activates the feature whereby if a family member does not have adequate personal balance to pay for something then the organizer's payment method (e.g., credit card) will be charged.
Read the document for the link to "learn how purchases are billed if a family member has Apple ID balance" ( How apps, content, and subscriptions from Apple are billed - Apple Support ). This tells you how it works. Essentially the payment method for Family Sharing is automatic:
1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple Account (Apple ID) balance, if any.
2. If a family member does not have enough personal Apple Account balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind). The Organizer's personal balance will not be used for purchases made by other family members. If it cannot bill the primary payment method, the Organizer will need to make another payment method the primary method.
Also note: "Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method." "Some subscriptions might not be charged to Apple ID balance."
Ref:
- Family purchases and payments - How to share purchases with your family - Apple Support
- How apps, content, and subscriptions from Apple are billed - How apps, content, and subscriptions from Apple are billed - Apple Support
- Check your Apple ID balance - Check your Apple Account balance - Apple Support
- Add a payment method to your Apple ID - Add a payment method to your Apple ID - Apple Support
Adding funds to your Apple ID balance:
- How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support
- Add money to your Apple Account balance - Apple Support
