Myst Port sits on the Land Bridge between Amaria AND Maldoria. It is a traders and travelers check in and inspection town that is literally cut in half with each Kingdom running and in charge of its own half. Both kingdoms operate customs houses here, and all travelers are expected to check in on each side and pay a exit tax. Southbound merchants pay the Amariaian side, and Northbound merchants pay the Maldorian side. Amaria charges a simple tax of one copper per hundred weight of goods, and Maldoria charges whatever the customs official at the time inspecting decides.
Both side of the border town are notoriously corrupt, with entire caravan loads of good being allowed to pass if the right official okays it.