If you were designing a download store that only offered one or two file formats, why in the name of God would you choose a format that isn’t natively compatible with ANY of the most popular jukebox applications or portable players?
It’s an open format, but that doesn’t mean it actually plays on anything. Why wouldn’t you just offer WAV or AIFF files, and let people convert them to anything they wanted once they were downloaded? Sure, they’re bigger, but they actually work on things that people use – and your customers have already demonstrated a preference for big files by shopping in your store.
Seriously. Get it together. You’re running a shop. Make it easy for people to buy stuff. Make it their choice to complicate their user experience.