Earlier quoted context omitted.
I once committed mail fraud when I was 17 years old. I had been using the return address as a destination address on envelopes to send mail to places for free without stamps. Finally got caught one day at the local branch. Postal Inspectors said I could face 20 to 30 years in federal prison.
That's insane. 20 to 30 years in federal prison for having defrauded the post of how many letters? Imagining you've been sending couple of letters a weeks at most, that's like what? $10 per months!?
That's just the maximum they use to scare people with. Federal sentencing guidelines aren't that severe and a first time offender with something as mild as using the return address to send a few letters would plead out to a few hundred or thousand dollar fine and maybe probation, say nothing of the fact that the OP was underage.
IANAL but to get 20 years they'd have to commit a massive fraud that ended in some serious damage like costing victims million of dollars with some other mitigating circumstances.