A fee was the standard charge for a service or access.
According to Jadzia Dax, in 2371, Quark offered Sisko to get in touch with several reputable interior decorators for a modest fee, after the Ferengi urged his profound disappointment in the accommodations aboard the USS Defiant. (DS9: "The Search, Part I")
When Captain Benjamin Sisko was looking for Odo at Quark's in 2373, the Ferengi bartender reminded Sisko that a war was going on out there and that he would help everyone forget his troubles – for a reasonable fee. (DS9: "Apocalypse Rising")