One who mails a bill (6)
I believe the answer is:
poster
'bill' is the definition.
(I know that bill can be written as poster)
I don't understand the rest of the clue.
Can you help me to learn more?
(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?" .)