I didn't bother after being quoted something around the equivalent of $60!
If you leave the now detachable cap on the pump at the filling station by mistake, it takes about 3 days for the engine management light to come on with an EVAP something or other warning.
Luckily, the filling station still had my cap when I went back there - along with about 100 others!!
And when I went to see my little man with the VCDS to find out what the light meant, he asked within a minute of plugging his 'puter in to the OBD port, "Have you been running without yer filler cap, mate?", and a minute later had cleared the fault, refused any payment and sent me on my way.
The local VW dealer would have wanted $150 just to find the fault code . . . and couldn't possibly have even looked at the car for a week!!