So we had a customer whose system we were upgrading to (among other things) implement a new corporate look-and-feel (since that customer had been bought by a corporate small-company-eater).
Among one of the things that they wanted us to implement was a 2% surcharge for customers paying by credit card. Yep, no problem, said the marketing guy / account manager.
So the question then arose: how do you check whether the card number which a customer enters is actually a credit card without actually connecting to that account and checking it explicitly?
We never managed to solve that problem, so what we did was provide a radio on the page: "Credit card" and "Debit card" or something like that. Both buttons led to the same code, but the credit card button added 2% to the price. So, if the user pressed "Debit card" and paid by credit card, there is nothing we could do to stop them.
Yes, that's right, we relied upon the combined honesty and naivety of the customer to press the right buttons.