“Mailman who happens to like collecting guns” is a solid character concept

Also known as Courier Six from New Vegas.

It's a little-known fact that about 60% of the mail on Logue Town is actually guns. The rest is bounty posters.

The morning delivery kills more people than the actual shootings.

"MAIL CALL!" *chuck*
"OH F-" *THUNK!!!*

This being One Piece, the logical next step was to deliver then guns via a handheld cannon, to minimise exposure. this worked surprisingly well until they stared delivering ammunition

No... at this point, you just gotta admit it, he is no mailman.

He is a actor playing in a cowboy show.

not a actor, he is the one in charge of the props.

D'uh, it's obvious, he is a mailmen, why should he only give WANTED posters? People also mail guns and he carry them to give them.