I seem to collect a number of, e.g., Visa gift cards that end up with just a dollar or two left on them. I would like a service that lets me use multiple cards

I seem to collect a number of, e.g., Visa gift cards that end up with just a dollar or two left on them. I would like a service that lets me use multiple cards to pay for a single online purchase. Some online stores let you enter multiple payment methods, but (in my experience) most do not. Ideally, this would be some sort of temporary credit card number that serves as an alias to one or more gift cards, drawing from them as needed, and automatically ignoring cards once their balance reaches $0.