Any credit/payment cards that you have registered on your iPad/iPhone, to enable online payments, are altogether separate from the payment method that your associate with your AppleID.
To set payment details for your Apple account, for payment of Apple Services (such as iCloud, iTunes, App Store etc.), go to:
Settings > [Your Name / AppleID] > Payment & Shipping
You will need to log-in to your account with your AppleID/Password - and from here you can associate the single Credit Card that you wish to user for charges made against your account - these if course include charges associated with Family Sharing purchased, for which you are accepting responsibility.
You may also find this Apple Support article to be useful:
https://support.apple.com/en-us/HT201266
I hope you find this information helpful in resolving the problem.