The planet only taxes imports. Exports are taxed through corporate and personal income taxes and/or sales taxes, because its domestic production generating income. Goods that go in and out the space side without crossing the landside border of the FTZ are considered 'duty free' by the planet. {This, by the way, is how 'duty free' works at airports.
} Any planet's FTZ would be encouraged by its ISA, but the planet would have a say in its size and scope. If the planet is paranoid anti-trade, then chances are that the starport will be very small and will not include a production zone. If a FTZ does exist ON AN IMPERIAL PLANET, it could be done one of two ways, inside or outside the XT Line. If it's inside the XT Line, it's under the control of the ISA (like Mora/Spinward Marches). In that case, Customs, Immigration, and Health inspections all occur at the XT Line. If it's outside Line, then the FTZ is under the jurisdiction of the planet. Immigration and Health clearances occur at the XT Line, but Customs clearances occur at the outer FTZ boundary.
I have to go back to canon to confirm Imperial policy. It was always my understanding that the Imperium DOES NOT CHARGE TARIFFS. Its policy mandate is to encourage profitable and extensive trade between Imperial worlds. It works to actively limit the tariffs charged by governments, thereby improving trade profits and increasing trade volumes. The individual ISAs are expected to operate at a profit, earning revenues from berthing fees, etc., but no tariff is charged on incoming or outgoing goods by the ISA.
The Imperium rules the space between the planets, not the planets. The ISAs seek to encourage trade, not make it more expensive.
Paul Nemeth
AA