If you have the locking fuel flap then they inevitably will break at the hinge, usually you are in a filling station and they embarassingly just fall onto the ground. Don't throw it away! Here's pics of how I replaced the damaged broken plastic hinge pin with a screw for a stronger job than the original

The flap is held on by two pins, one is metal, the other plastic, no prizes to guess which one breaks

To remove the existing hinge assembly from the car then just pull back the wheel liner enough at the bottom to gain access to the the two nuts by sliding your hand in behind the liner and remove the 2x 10mm nuts there and remove the whole flap hinge and cap assembly. NB there may be some packing washers on the threads, note now which goes where as they align the cap mount. You can leave the cap still connected but plug the filler hole if your car is on the street otherwise a passing smoker may get a surprise:)

Best to detach spring for the moment to get it out of the way

File flat remnants of the old plastic pin then choose a suitable size self tapping screw and carefully drill a small pilot hole and make sure it's going in straight. I found it best to keep drill steady and move the hinge onto it by hand with drill at VERY slow speed. Then with drill of right size so screw will grip well carefuly drill to actual size you want and slightly deeper than the screw depth

Screw the screw in to cut the thread and make sure it threads in straight

Now reassemble everything and remember you want the screw tight in the thread but not tight on the hinge as it is a hinge. Reattach the spring and reinstall

And, of course, because it's the existing flap then no spraying to do for a perfect match

Cost 2p screw and 30mins labour. Fixes don't get much cheaper than that (otherwise parts would be £30 plus cost of respray )