One who mails a bill (6)

I believe the answer is:

poster

I'm a little stuck... Click here to teach me more about this clue!

'bill' is the definition.
(I know that bill can be written as poster)

I don't understand the rest of the clue.

(Other definitions for poster that I've seen before include "bulletin board participant" , "Bill or placard" , "Paper advertisement for topers" , "Person who sends" , "surfer who leaves messages on boards?" .)

I've seen this clue in The Globe and Mail.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.