Sorry this has happened to you Lauravt. Your seller's response sounds desperate to me - going from denying it's a fake to creating a line about you refusing a tracking number in such a short space of time suggests to me she knows she is on dangerous ground. I would send a calm reply using the Paypal console saying that you will certainly return the bag using a tracked and signed for service if she is now willing to offer a refund or alternatively you will continue with the authentication process and asking which she would prefer. I am willing to bet that might focus her mind even further!
It sounds to me like she has dropped the authentic/fake defence and is now simply looking to wriggle out of the claim, so I imagine you probably won't have to seek formal authentication after all.
Let us know how things go and if you want any more input.