Airmail: Lightning fast mail client for OSX

I was curious if there had been any discussion about this. I suppose not. Recommended in the "Mavericks ruins Mail.app" thread[0], I was surprised to find it only costs $2. Given the lessons of Sparrow[1] (which sold for $3!), this doesn't seem sustainable.

What's their business model? Is it entirely based on $2 upfront sales with no sustained support? Because I cannot imagine that lasting long.

[0]. https://news.ycombinator.com/item?id=6601202

[1]. http://davidbarnard.com/post/58970496119/the-sparrow-problem