I got a funny response when mailing regular letters for work. They are now putting the regular letters through a little slot and it if is more than 1/4" thick they charge you the large envelope flat rate of $.88 instead of $.44.
It used to be that as long as you could stuff it in the regular #10 envelope you got charged the envelope rate not the flat rate.
They said it was because they're broke and need to start checking the size of the mail going out not just the weight.