Crossword Puzzle One going to the post office

Clue: One going to the post office

Post office activity, or a hint to this puzzle's theme (LETTERSORTING with 13 letters)
Lines at the Post Office (ADDRESS with 7 letters)
Post-office stamp (AIRMAIL with 7 letters)
Post-office alert letters (AKA with 3 letters)
Post office poster letters (AKA with 3 letters)
Mil. post office (APO with 3 letters)
Post for fastening towlines (BITT with 4 letters)
Post-office offering (BOX with 3 letters)
Post office rental (BOX with 3 letters)
Post Office purchase (e.g. BOXNUMBER with 9 letters)