Publications are not shippable for electronic versions, yet I don't see where that's controlled on the edit screen. How?

Ubercart doesn't have built-in the ability to make one version shippable and the other not. So I ended up writing some custom code that says:

If a SKU begins with K or E then treat it as non-shippable.

The code resides in sites/all/modules/worldwatch/worldwatch.module