there: kudos to you for executing exact the idea I had as early as 2010! By any chance you could write an article on your experience with sells and marking? This is what most interests me! I am working on a project that will utilize Pushover so soon I will be a paid customer :)
Also, about your revenue model. How did you calculate that $3.99 would be sufficient to make profit AND give service to users on "all you can eat" terms? Wouldn't it be wiser to give one year for free (thats plenty of time to see whether I need and like your service or not) and then charge $9.99 per year after that? I mean, if this grows, I can easily see people eating up more bandwidth than this one-time $3.99 payment can bring into your pocket. If your TOS (sorry haven't checked) does not explicitly saying that in the future you may charge more, I think you may found yourself walking on a thin ice.