Recommendation against requiring Interval

I think I understand the use case that you're intending to require Interval for - publishing a node for a limited amount of time and having it unpublished later perhaps? Whatever the precise requirement, I'd strongly recommend not making that a requirement of the core module. Limited time publishing is not a requirement of the core functionality, which is primarily to complete checkout with a product referenced to a node and be able to react to it upon checkout completion / payment. Whether that involves limited time publishing, immediate perpetual publishing, or nothing to do with a publishing workflow at all should be up to the site builder.

That said, it might make sense to bundle a Feature inside this module that demonstrates what a dependence on Interval can do for you, but it shouldn't be something everyone who wants node checkout functionality should have to have.