I have a few suggestions concerning the Tubepress licensing policies.
1. On this page : http://tubepress.com/pricing/
I suggest you include a statement about the number of websites that a client can use the plugin on. This way a client who has been using the free version before will be able to make a judgement whether it is worthwhile to pay $39.96 per year for use on a single website. Even I only now realised that I am allowed to use the Pro version on a single website only. That's a real bummer and I am sure many more of your clients may feel the same way if they find out only after paying for the license. In my own case I may be actively searching for other solutions during this coming year and may not renew the 1 website license after 12 months.
2. A further suggestion I have on this subject is perhaps you can offer a "Gold" Pro version for $39.96 per year, where you limit usage to say 10 websites. If a client wishes to use the Pro version on more than 10 websites you can offer a "Platinum" Pro version at a higher license fee for unlimited websites.
3. You will need to have a system whereby every client will have to register their websites so that you can keep track of the number of websites on which they are running the plugin. Say someone who bought a "Gold" Pro version has hit the threshold of 10 websites then they will not be able to use the plugin on an 11th website unless they upgrade to the "Platinum" Pro version or buy an additional "Gold"Pro version.
Just some thoughts which I hope you will consider seriously.