Most places, if they have a minimum for a credit card charge, will have it posted. If not, you can ask, "Do you have a minimum charge for the use of a credit card?"
Which is correct? You don't ask but for the sake of argument, which is right?
Is there a minimum credit card charge or can you pay whatever whichever amount? Is there a minimum charge for the use of a credit card or can you pay whatever whichever amount? Is there a minimum for a credit card charge or can you pay whatever whichever anount?