If you pry off the metal plaque and peel off the sticky label, it'll be a generic bag and you can re-donate it.
Ignorance of the law is no excuse for breaking it.
Thrift stores will
claim that items are donated and because they're just reselling donations. To give the benefit of the doubt, they may or may not know the law but that's their issue. They claim not to be selling as "authentic," but that doesn't relieve them of their requirement to abide by the law.
As for returning, tell them that the penalty for selling counterfeit items is $250,000 per item. They should have verified authenticity before putting it on the sales floor and if they didn't do that, they were in the wrong.
If you paid by credit card, you can open a dispute with the c.c. company.
Buying Counterfeit Goods: Laws and Resources - Consumer Protection
From the above: (While they might dispute use of the word "trafficking," that's semantics. For the quantity of fakes being sold by thrift stores - knowingly or not, it IS trafficking!)
The maximum penalty for first-time offenders trafficking in counterfeit goods is 10 years in prison and a $2 million fine. For second-time offenders, the penalty is 20 years and a $5 million fine. In addition, if a corporation traffics in counterfeit goods, it can be subject to a fine of $15 million.