I understand your point of view, but digital download is a “product” that’s not so mobile-friendly due to the reasons I’ve explained above.
By saying “you don’t need to care about digital downloads” I mean that you can’t assure full compatibility with all your customers, unless you sell certain type of files like PDF or images (with some limitations).
PDF documents can be opened directly in browsers and most of the times can be saved, for images you need to pay attention to the size, really hi-def images are quite big both in KB and pixels, browsers on old mobile devices sometime can’t handle them correctly and crash or don’t display them.
You say that you create ZIP archives of your MP3s but ZIP files can’t be opened by iOS.
MP3 is already a compressed format so you don’t need to zip it, but anyway you get a single ZIP file to download instead of multiple tracks (if you have more than one track).
Leaving MP3 files as they are, the customer must open each track and, as far as I know, Safari Mobile doesn’t let you save them into your Music app.
If I were in your shoes, I would use ZIP files and if a customer complains that he can’t open it with his mobile device, I would reply to use a computer or install a 3rd-party app that can handle it.