It can be either the canister or the valve. It's easy to replace both and they're not very expensive, just messy because they're located behind the passenger-side rear tire underneath the car and road filth accumulates on them.
I have a vehicle that for years would trip the overfill shutoff at the gas pump for unknown reasons. I found out that if I turn the filler handle upside down, I can pump at nearly full speed without tripping the shutoff. Not sure if that will work for you temporarily until you find out the issue.