Ugh! I'd be just as angry. They have the 'report a fake' feature but they do zero about it; thus the fakes are allowed to proliferate to a disgusting degree.
When you say the bag was authenticated, do you mean Posh authenticated it? If so, they failed to do it properly, causing you to have to pay to prove them wrong. If the seller had a "certificate" from some crackpot 3rd party authenticator, then you may be out of luck, sadly.
Regardless, it takes all the fun out of getting a new bag when it winds up fake and you have to go to such an effort to remedy it.