I had an issue like this with Amazon back when their marketplace was a new thing, I think around 2000 or so. I ordered a used iPod (the original thumb-wheel one

I had an issue like this with Amazon back when their marketplace was a new thing, I think around 2000 or so. I ordered a used iPod (the original thumb-wheel one) from a marketplace seller and I had a $100 Amazon gift card to put towards the order. Their checkout took the gift card code and it appeared that my total was being split between the gift card and credit card, but it ended up charging the entire amount to my credit card. Not the end of the world or anything, but I guess it shows that checkout processing is non-trivial and even the big guys can screw it up. Or maybe especially the big guys can screw it up by having a large number of confusing discounts and policies to apply.