I always spent the $16 to send it overnight. That way it required a signature, got confirmation, and showed the adcoms I meant business. Ha ha, it was probably a waste at a lot of schools. However when I was able to finish a secondary in 1 day and return it the next, that was a 2 day turn around. Impressive I think so.
Most of the time, I just stick it in the snail mail. It's not THAT slow for just a standard envelope. I mail it priority if it's bigger, just to be safe. I've always included a cover letter just to make sure they're not overlooking anything like a check.
I used certified mail for all first-class mails and delivery confirmation for all priority mails. I sent all mid & east-coast schools using priority and every west coast schools using first-class. I am from CA. so far i spent 3500 on application alone, $2 extra is nothing.