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It is almost certainly the case that there is no readily available mechanism for easily changing the price on credit card only machines without changing it on cash + credit card machines. Presumably changing the default pricing structure would have to change both, which would cause the problem that either a great many machines would suddenly be providing $0.95 worth of change for everyone paying with a $20, or everyone would need to start carrying around nickels.

Clearly it is more work than a simple price change to create a new user interface that asks whether you're going to be using a credit card or not before displaying the prices.