I just stuff it all back in the outgoing slot
... or you can put it in a mail box.
When I got out of school, I worked there at Christmas a couple of years. Loved it. Yes, there are a lot of slackers ... especially at the main branches ... like downtown LA.
But, a lot of exercise delivering door to door, met a lot of nice people. It was nice all the way around.
Stuff the larger return envelopes to the brim with other junk and send it right back.