I would file a claim with PayPal FIRST before contacting the seller. Especially if you are SURE it's fake. No need to give her a heads up. After filing the claim with PayPal email the seller and tell her you are shipping back the FAKE bag and that you want her to refund your money when she receives it.
I would also call your credit card company and start a claim with them. You need to cover all of your basis at once, not one at a time.
When you ship the bag back make sure to include signature confirmation, or return receipt or some other type of shipping that allows you to get a copy of the signature of the seller. You will need this for Paypal. I recommend signature confirmation because it will also track online and you can give that number to PayPal right away.