Your credit card is needed so that you can cover the cost of prints while giving your clients large discounts or free prints.
For example, your client orders a 4x6 print. It's normally $3.00 + $6.00 shipping, but -they use a 100% off the subtotal discount code. Here's what would happen:
They would only get charged $6.00 for shipping
They would pay nothing for the print because of the discount
You would get charged for the cost of the print, $0.78
Note that you would only get charged $0.78, not $3.00. That's because you're charged for the actual cost set by the print lab, not the price you set in your catalog.
What if your client uses a 100% off shipping discount code?
They would only get charged $3.00 for the 4x6
They would pay nothing for shipping because of the discount
You would get charged for the cost of shipping: $6.00