Family sharing in iCloud won't let me use my own credit card

The short answer is only one person, the Organizer, can (and must) have a card, but the Organizer can avoid having this charged by others by asking them to make sure they have adequate personal balance to cover their purchases. Other family members can only pay for things by using their personal balance or by letting the remainder automatically roll over to the Organizer's payment method (such as a credit card but not the Organizer's personal account balance).

The alternative is described in this document: 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 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."

=Full details=

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 Apple cannot bill the primary payment method, the Organizer will need to make another payment method the primary method. With Family Sharing the Organizer always has to have at least one functioning paymenet 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

As I already said, it is a clever way of ensuring people don't abuse the system. You can still make your own purchase. Either pay for it using personal account balance from adding gift cards, or let the other person pay for it and then give them some cash for the payment they made for you.

If you think you have a better method which will ensure content is validly shared between family members (and not a pretend family) then I am sure Apple would consider it. Product Feedback - Apple

But this limits functionality of a true family where there are 2 adults with their own credit cards and purchase needs. Apple really needs to address this issue as it's been a request for years.

It's a clever and deliberate method which functions to restrict sharing to people who would have a "family" level of familiarity and trust instead of just any 6 people who might have met at a bus stop and are trying to get some free items.

In order to get around this, building off of the fact that you must have funds in your own account in order to make a purchase without charging your parents’ card, what you can do is buy an Apple gift card for however much off of Amazon, send it to yourself, add it to your account, and then make the purchase. It’ll pull from your account, not your parents’ card.