We know that instant transfer from one storage to another is expensive.
With keeping it that way, because otherwise there wouldn’t be any meaning to have split storages, there can be a daily caravan system between the cities. Say that everyday at 08:00 AM, set goods are transfered with a low amount of fee.
It would also be nice to have an interface like “Transfer all Irons to Settlement Y every day but lock/keep 200 in source settlement”
It’s a draft idea. Fees can be increased if the target or source settlement does not belong to your faction but it should be kept cheap comparing to instant transfers.