An import permit was a document that allowed someone to import a commodity.
In 2373, Quark brought a shipment of Regalian fleaspiders aboard Deep Space 9 for Dr. Julian Bashir without an import permit. Bashir explained to Kira Nerys that he had ordered them in order to synthesize a drug from their venom to improve her circulation, but hadn't realized he needed a permit.
When Kira suggested they just fill out the necessary paperwork, Odo explained that the Ferengi had also illegally imported Regalian liquid crystals along with the fleaspiders. Having to head off on the USS Defiant, Kira left both bartender and doctor to handle their legal problems themselves. (DS9: "The Ship")