How about Shareware style nag screen?
I have donated in the past to your efforts and applaud your continual striving to improve this skin. It is far and away the best thing out there.
How about offering a free version with a Nag Screen at every startup? This will allow those users who cannot afford $5.00 (but they have a car and a carputer - I call that cheap) to use your software, with a constant reminder that they should pay up.
Also, how about allowing those who want to generously donate more than $5.00 to do so and provide some type of perk.
You will also have to look into the licensing agreement of the plug-ins you use and code to ensure you are not liable for compensating someone else out of your pocket...
Good luck, Marc