Hi Susan:
We have an identical situation and we do the following:
1. I have set up a new bank called (in my case) "Mastercard" and assigned a GL account to it, lets call it Acc. 1020.
2. The invoices are set up in the payable module in the usual fashion.
3. When I phone a supplier and pay the invoice by Mastercard, (and note on the invoice how it was paid, who I talked to,etc) I then pay the invoice in accounts payable by writing a "cheque" on the Mastercard Bank Account. When this cheque is posted and then retrieved the entry automatically goes to my Mastercard "Bank Account", account 1020 in the GL.
4. Then, I write a cheque on my regular bank account to pay my mastercard, selecting 1020 for the distribution code.

Hope that helps. It works well for me.
Ken French
250-861-8884 Ext 111