Julia, if you get a chance, can I check out your ec_paypernode module on your site to see how it works? Not sure what you'll need to do to get that working. But I'm looking at ec_paypernode and thinking of adapting some of their code for an Ubercart module, but that UC version will be more based off allowing users who purchase a Product Creator role to be able to do just that. That way we could have the module be fairly independent and only rely on the user having the proper permission. From there you can set # of allowed nodes to create, expiration, etc.

Also, what were you looking at with regards to tying the paypernode module into Workflow-ng? I would imagine the Payment complete features and that sort of thing. That's probably a good idea, looking ahead...