I think it's because you're trying to do something that's not currently possible.
You can't currently sell digital products and physical products at the same time, using the Market Theme shopping cart buttons.
This is because you can't use the e-junkie cart checkout, and the Market theme shopping cart at the same time. They are different systems.
We recommend using the E-junkie system for digital products, because it offers dynamic download URLs. This provides a limited time available for download. If you used Market theme for digital products, the URLs would be static, and you'd have to rename them periodically to keep the download URLs from being shared.
I hope this makes sense.