It'd be an envelope. Also, if you've lived elsewhere and had friends there, just say the letter is from them. :P
I've lived in one place my whole life, and even if I could use that excuse my parents are ridiculously overprotective and would ask a gajillion questions.
I'll see what I can do though. I might be able to get it from the mailbox before my mom gets home from work.
I'll PM you my address if I think I can get to it (cause my steam profile uses fake info as do all my other profiles).