Caravans are a huge part of the setting, particularly in Dragon Pass. Especially caravans going through Sartar once it is liberated. Beginning in 1627, the new Prince encourages caravans traffic and places them under his protection, in exchange for a toll based on the value of the commodities carried. I suspect the toll is collected by the Issaries temple in any of his cities, but caravans that have paid the toll are under his protection from those Sartarite and Praxian tribes that respect the Prince’s authority.
These tolls are probably the bulk of the Prince’s revenues, but quickly give the new Prince great financial resources. Which make it a lot easier to buy the loyalty of tribes and mercenaries.
“Lucrative revenue streams!”, shout the warriors who get paid with it. “Need to track it!” worry the Lhankor Mhy scribes. “Respect and importance!” say the Goldentongue priests, smiling.
I can see Esrolia trading freely through Argrath’s dominions but where are the caravans going? Aren’t the Lunar provinces going to restrict access to and from the war-zone pretty tightly? Why would they? Lunars want luxury goods too!
That just means that a successful caravan operator needs to be able to work with Argrath’s people and with the local Lunar officials. One more reason why Issaries tries to stay neutral!