here is my experience: I emailed the seller who denied that it was a fake. so I contacted paypal and opened a dispute and then escalated after 2 days as comms with seller was going nowhere. the paypal employee told me to get it authenticated and send the letter to them asap as then I am ahead of the game. i got the letter but never had to send it as the seller just ignored the claim until the deadline was up. I got my money back instantly, so it took about 15 days all in all.
I would contact the seller and file a dispute/claim right away, they can always refund you and then you can close the claim. but if you have a lengthy discussion with them they may move their paypal funds and then there is nothing to get. also, are they covered up to $2000? don't know how much you paid so this would be something I would want to check out.
if they ask you to return the bag i would still get it authenticated first to be on the safe side. and I would let them agree to a refund only through paypal. also, if they ask you to close the claim in order to get your refund, do not agree to it - if you do so, no way to open the claim again. before you send the bag anywhere take a lot of pics of it to prove you haven't damaged it, ideally you do so with the date on the pictures. if you return it please only do so via recorded delivery.
OH, one more thing - you don't actually have the bag yet, right? in that case I would wait for it, the reason I am saying that is that if they don't send the bag you will have to report it as not received in which case you can't change any paypal dispute/claim to it being a fake! well, you would always your credit card company as well. good luck