Our extension, Gmail Auto Pop, uses ExtensionPay to handle subscription payments. We started at $5/month, but have recently changed to $12/year. Extension users are so used to "free" that some users who were just trying it out balked at the $5/month price. Serious Gmail users thought it was a bargain!
We did some work wrapping up state changes (TrialActive, TrialExpired, PaymentFailed, PaymentCanceled, etc.) to make the code react more quickly than the standard "Check subscription on browser startup / new Gmail tab", and to make the user messaging more robust. We'd be happy to share some of the code we came up with, if anyone is interested.
Is anyone else using ExtensionPay, or another payment processor? What technical challenges did you face? Also, what pricing model did you use, and how was that received?