Q: Can you pay with a different credit card when you use family sharing?

Hi there AodhFFXI,

I would recommend reviewing the information in the articles below.  Keep in mind that all purchases made by members of the Family Share are billed to the organizer's account. 

Family Sharing

http://support.apple.com/kb/HT201060

To get started, one adult in your household—the family organizer—sets up Family Sharing, invites up to five additional family members, and agrees to pay for any iTunes, iBooks, and App Store purchases they initiate while part of the family group. Once family members join, the features of Family Sharing are set up on everyone’s devices automatically.

Once you’ve set up Family Sharing, all the songs, albums, movies, TV shows, books, and apps ever purchased by family members are immediately available to everyone else in the group. And, of course, so are new purchases.

All new iTunes, iBooks, and App Store purchases initiated by family members will be billed to the family organizer’s account.

Family purchases and payments

http://support.apple.com/kb/ht201079

If you don't wish to be billed for purchases initiated by a specific adult family member, you can remove that family member from the family group.

-Griff W.