There is an increasing trend now for many FBOs to charge a handling or ramp fee even for light singles, almost always waived if you buy some fuel but the markup is gonna cost you a tenner on average. On the other hand you can get your money's worth of coffee, popcorn, Internet & wx access, magazines, etc, during the stopover! I've even spent the night in an FBO snooze room and they'll often have a car you can take for a couple hours at no charge.
As part of my flight planning I browse the comments on fuel prices, fees and perks on the airnav.com site to select which FBOs seem to offer the best deal (there's often a choice - infact often a choice of airports too!).
It's also possible usually to negotiate, if you apply maximum charm at front desk they sometimes are happy to waive things like overnight parking if you're stuck due to weather, or waive the ramp fee / fuel purchase if you beg, "awww but I'm just gonna be here 10 mins!"... or... "ramp fee? hmmmm, we don't have those in Britain, do I really have to pay?" is often just as acceptable as Mastercard
You can also try parking the plane discretely then striding through without registering. This is all particularly easy at FBOs which are busy with jet traffic and the staff have more lucrative customers to chase up!
Another new trend at some airports and FBOs is the need to show your license & photo ID, sometimes when you arrive, or more likely when you want to go airside again, so don't leave it in the aircraft, just incase.