Tameable pigeon or hawk or some other bird for sendin small items to postboxes, could be used for long range tradin without havin to use knarr.
Postboxes can have password put on them, like wilderness beacons, for short time and the bird can be told to go to postbox with correct password. Upon arrivin to postbox it puts the item inside and leaves back to sender. Or with specific item postcard which would work like "pay x item(s) to open", set by the sender. The bird would wait on the postbox for the postcard to be paid before goin back to sender. Maybe birds could be intercepted while in flight so its not completely safe way to send items/trade?
Bird inventory could be 2x2.
Also, would probably need some falconry related building and items to tame and keep birds.