Hi there Funkle,
If a user bought a model, he gets the download link and he can download the file a few times, they definitely don't get charged twice. Also, if the buyer downloaded the zip file, shouldn't he get all the formats in one zip file instead of a need to download it many times?
If what you're trying to say is that you want to sell the same model but charge for different formats, then it's a whole another case, although I think that would also raise a few eyebrows..
If this is a different case, try to explain it, as maybe I have misread it in some way.
Regards,
Eddie, CGTrader