A couple of years ago I did a high altitude balloon launch with an APRS tracker using an Arduino. Many have done the same.
Shortly after that I ported my GPS and APRS code (including the AFSK code) to Teensy 3.1. I got it to work, but never used it for anything. Code is here.
It turns out that I'm just starting on a project to do another balloon launch using a Particle product. I intend to port the code to the Particle environment soon.
As for a transmitter, I'm using the Radiometrix HX1. I bought it some time ago, and I'm not sure it is available in the US anymore.
Before I found the HX1 I simply fed the AFSK signal into a cheap HT (Baofeng). Sort of ungainly, but it worked.
Anyway, let me know if you want to pursue this any further with me. Happy to kibitz.
-- Rich KC3ARY