Generally speaking the "CoC" gets accepted but like it has been said previously, its hit and miss.
The fuel truck driver always asks if the flight is Commercial or private and when you say its Commercial they ask to see "AOC". Given the "CoC" has Certificate of Commercial Use on it, the drivers expect to see "AOC" - No "AOC" no commercial fuel. I think if there is a bit of education to the fuel companies as to what is Commercial and Non Commercial then the "CoC" may go further.
Depending on the driver's interperation of the "CoC" depends on whether he will give you the fuel at the commercial rate. I have had drivers go off to speak to their offices for approval and they always want a copy, so always a good idea to carry copies onboard.
I have had good and bad with the "CoC" - Managed to get it accepted in Greece, Nice, Le Bourget , Germany Italy and Spain. - Again its hit and miss!
Keep trying
3legs