Crossword Puzzle Labels at the post office?

Clue: Labels at the post office?

Post office activity, or a hint to this puzzle's theme (LETTERSORTING with 13 letters)
Attained, as as office (ACCEDED with 7 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)
Big brand of office labels (AVERY with 5 letters)
Post-office offering (BOX with 3 letters)
Post office rental (BOX with 3 letters)