Being Fired VIA Certified Mail

I'm on day 2 of hell at work so this makes it that much more aggravating. I worked for EasCare for almost 6 years as an EMT & Dispatcher, and I'm not really shocked by this but it just chaps my ass.
Apparently EasCare requires their 'per diem' employees to work atleast one shift a month, however there is no set schedule for per diem and they are supposed to call you to fill in a shift. They never called me. I didn't want to work a set day of the week because I was afraid it would interfere with my job at the PD or any possible $$$overtime$$$, but if they were to call me and I was availible I would've worked. So I went to the post office today before work to check my mail, and I have a manilla envelope with a certified mail reciept on it with my 401K info and a letter stating that for " Those not employed by the company that would like to cash out their 401K" .... I was like "Um, I'm being fired?" Not really fired I geuss, "let go" since no one informed me of this rule of per diems having to work 1 shift a month. Its kinda like "Surprise ! You're Fired!" now sign this reciept .... fuckers.... Oh yeah they sent it to my old address in New Bedford even though I had told them repeatadly that I had a po box ..... and the original envelope was opened .... but I'm pretty sure it was done by eascare .... In any event I'm pretty freakin pissed that they just let me go and did it via certified mail, shit heads. Working one shift a week now would be pointless besides, to drive alll the way to Dartmouth would end up paying for the gas and that's about it..... Oh well, no more stair chairs for me, I'll stick to babysitting the shitheads instead of carrying them up stairs.