After parking overnight at APC; a general aviation pilot discovered fee envelope attached to the gear strut with rubber bands. The aircraft has retractable gear and the reporter believes envelope could interfere with gear operation.

Narrative

I parked my aircraft overnight in the transient parking area at Napa County Airport (APC). Upon pre-flighting the aircraft for departure the following day; I noticed an airport parking fee envelope attached to the left main gear leg with rubber bands. This envelope was attached to the underside of the main gear leg and was not obvious except upon close inspection. I presume that this envelope was placed on the aircraft by airport personnel in order to collect the overnight parking fee. My aircraft has retractable landing gear. If this attachment had gone unnoticed; possible interference with the landing gear retraction mechanism could have occurred. I am concerned that airport personnel are attaching inconspicuous items to aircraft without the pilot's knowledge. Even if the items are conspicuous; I do not believe this to be a wise practice. Airport personnel cannot be knowledgeable of every aircraft type and could unintentionally affect the operation of aircraft systems. I believe that airport personnel should refrain from attaching items to parked aircraft.