Suggestions for field delivery:

Retainers. You send your retainer out to deliver the package to the recipient. During their travel time they would be unavailable.

Mail Outpost. At each camp there would be a mailbox. Chocobo carriages would be sent out and deliver mail at a set time each in-game day.

Air Drop. Airships flying overhead could drop packages off the side for adventurers on the ground.

Goblin Drop. Similar to air drop except a goblin jumps off the edge of the ship with a parachute.

Obviously these should have an incredibly high delivery cost though, so that people wouldn't use it for just anything.