To sign "Post Office," just spell "PO."
The simplest way to indicate that you are involved with mail delivery is to
sign, "I WORK P-O."
MOST POST-OFFICE WORKER THIN, WHY? (Why are most postal employees thin?)
Here are some other ways:
LETTER + WORK + AGENT (Mail worker)
LETTER + CARRY + AGENT
To sign POST OFFICE just spell "PO."
for "stamp" see the MAIL page