Yeah, don't get me started on this...
In the GA context, having to clear Customs is a must even if just refuelling AND if one can pay the bowser so one does not need to leave airside.
I have been refused landing at Padova (Italy) on this, from a short final
I have also had my flight plan to Zaragoza (Spain) summarily cancelled even though they were told (by my departure ARO, on the phone) that it was a fuel stop only
In both cases, lack of PPR was cited, although the circumstances were a bit suspicious as regards some individuals' conduct at these places.
As far as I can tell, fuel stops without clearing Customs are done by prior arrangement with a handling company, who presumably square it with the local Customs office. It is frequently done with bizjets, and I even had it as a passenger in a 747. But, despite enquiries, I have never got my hands on an example of how to do it.
One point which someone picked up was that if you did do such a landing then the next landing point would have no idea that you did not clear Customs, and this could be used to circumvent the universal need to enter each country (or Schengen) via a Customs airport.
So, for example, you could depart the UK, refuel at Caen LFRK, and fly to anywhere in Schengen and everybody there would think you entered Schengen properly at LFRK.
Therefore, my guess is that this can be arranged only for flights on which the onward destination is another international airport. In the above example, refuelling at LFRK should be OK if the next stop is
definitely going to be say Biarritz LFBZ. This would be open to obvious abuse because you could just divert to some farm strip in France and there is no apparatus in place to detect what actually happened at LFRK.
Why is this available to bizjets but apparently never to GA? My guess is the answer is that in general nobody wants to be ar*ed with providing services to GA, anywhere. Money paid to a "handling" company can however be used to "lubricate" (sorry I meant "facilitate") all kinds of favours, and jets are big income to most airports and their operators are well looked after. Also, a GA flight is much more likely to divert to somewhere small and local than a jet which is departing on a long flight somewhere and which is unlikely to play any games. But I am really speculating.