Paying for my own purchases in Family Sharing

YES!!!

Fix for what ? How purchases are billed is working as designed, if you would like it to work differently try leaving feedback to Apple.

All you need to do is purchase iTunes gift cards and keep them on your account. Then it will use these before using your mom's credit card.

Have you actually gone through with a purchase to see what is charged for it ?

Have you gone through with any of the purchases, does it actually charge the organiser's card ?

I have. And it does charge his CC instead of redeeming the iTunes balance.

Have you gone through with a purchase and got him to enter the card's code ? Is that card actually charged (except for a temporary store holding charge) for the purchase ?

If the account is in a Family Share you can't use your own credit or debit card. You can use iTunes gift cards, I use them for all of my purchases, including iCloud storage

It is working as designed, Apple are not saying that it's a feature, and it's not a bug. If you don't like the current design use the feedback site to leave feedback to Apple

And ? I'm not in any way responsible for how Family Share works.

I just want Apple to know

Apple are not here, these are user forums. If you want to leave feedback to Apple use the feedback link already posted on this thread