No, you're right. This is a common problem, sadly. Those "dumb" websites have no parameters for enlargement, they just look for numbers of pixels.
Everyone who sells digital files face this problem from time to time. It's yet another reason why you should be aiming to shift your business model towards printing, not selling digitals.
For now, find out what size they are trying to print, and which files, and resize them specifically and send them again.