AFAIK, the only form of payment is charge to the cabin account with a Medallion scan or manually entering cabin nbr and verifying name in the system.
Idea: If sailing with a friend, bring friend and have it charged to their cabin. Go to GS and ask them to make a payment on friend's account using your credit card.
I presume you are concerned your mate sees it. In my case, I see and pay all the credit card bills and details with online banking, so I'd know about a charge anyway and may or may not pick up on something.