Had a couple of occasions where they have got the customs guy involved, In all cases customs have asked me to refile type of flight from G to N. The problem then went away.
I've had a fair few standoffs over this, I just refuse to budge or sign anything and [quite seriously] threaten to take them to court if they don't recognise it (fuelers, customs FBOs, and all) and so far it's always been resolved.
On one occasion the fuel company had a change of heart. It was a day we could have managed without and I told them to de-fuel the aircraft and take it back because I wouldn't be paying for it with tax. They decided just to recognise the certificate instead.