Don't forget experimental licenses. You can literally do anything you can talk the FCC into permitting, and they're generally nice people if you apply in good faith (like, apply for a supposed R+D license to transmit wideband voice FM on 101.1 MHz 24 hrs a day and they'll laugh and tell you to go away, but for genuine serious R+D requests they'll bend over backward to help)

I was not directly involved in legal paperwork stuff but my understanding is a short term STA costs a varying amount depending on service code and all long term form 442 cost sixty bucks. I also recall the 442 is about a tenth the paperwork required, so I don't really see a point in requesting a STA for truly experimental work. Of course you can get a STA for literally any service the FCC regulates, whereas form 442 are only for legitimate blue sky experimental research.