It can go in a regular mailbox, but the postage depends on weight. I'd figure that for a 10x13 envelope with up to 4 or 5 sheets of paper, you'd be fine with 2 first stamps. Anything more than that, and you might want to consider 3 stamps. I worried about postage too when I sent my applications, so I always took them down the post office and had them weigh the envelopes and put a exact postage printed stamp on the envelope. Even for the small envelopes that only had 1 sheet in them where 1 stamp was more than enough, I always put 2 on those, not because I was worried about postage, but I wanted a redundant stamp on there in case one fell off 🙂