Me neither. But then again most don't charge a fee for parking away from the main ramp and not using any services.
I dont think OP parked "away from the main ramp". . if they did - the FBO wouldnt even know about it or thought to charge. Im willing to bet that the OP clearly knew they were parking on the ramp at an FBO. Claiming you didnt know is completely different than clearly you didnt - as the assertion is more - clearly I did know and I was hoping to get away with it.
I don't think you should be charging a fee if you aren't posting it clearly online, in your business, and on the ramp where the "services" would be provided. The only reason not to do so is to trick people into owing your fee. If it were posted, it could be avoided and we wouldn't be complaining.
You go in to a restaurant and you dont know prices until you ask someone. Same here. Sometimes its posted online, sometimes on the door/window. Sometimes it isnt. They didnt trick anyone in to owing a fee - as the OP already has a card on file and has been there before and paid it. The only "trick" being played (or attempted) is that he was hoping to get out of there without going to the front desk after being on their ramp. If you want to try and pull this off, go and park it where they have hangars and maybe part it out of the way and not blocking anyone. That might be more successful than parking at an fbo and hoping not to be charged because you never checked in.