My license was suspended for 6 months (WRONGfully might I add!! no, really) apparently, and 3 months in to the suspension that I didn't know about I was pulled over.
The cop notified me that my license was suspended and to either get it taken care of, or stop driving. You're supposed to send your license in or something though when it's suspended, and being that I still had mine, perhaps that's why I got the reaction that I did (which wasn't much of a reaction at all, and he let me off going 9mph over the limit
).
I can't answer the letter part, since I never got one though. But if you do have serious problems, call your state representatives office and have them deal with PennDot.