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I'm using zencart very happily, although I haven't yet integrated it with xoops.
Someone already has created a XOOPS module for it. Zencart handles music downloads, in fact it installs with a demo version. The code is being very actively developed at the moment with a stable team of competent coders and it looks clean neat and well designed. OTOH it could do with some of the features that makes XOOPS easy to use such as a better implemented module install methodology.
I'd try out the XOOPS zencart module and if it falls short either get the developer to fix it your needs or visit
zencart website and pay the developers there to clean it up.
Lee
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Complexity? Its just lots of simple things connected in lots of simple ways...