hey ya,
something equally weird happened to me a little while ago. i filed for non-receipt and the seller sent the bag just before the claim was decided in my favour so I received it after I got my money back. the weird thing is that she didn't respond with the tracking number in time (never responded at all) which i just do not understand. afterwards she told me that she had another buyer so if I didn't want it, i could send it back.... I guess she was sorry from the start about selling it to me at a low price.
anyway, I phoned paypal and they said that if the seller could prove delivery to me, then she could get her money back. it would have been difficult to prove for her but frankly I couldn't sleep well for about 3 days - so i sent her the cost of the bag in the end and took shipping as a deduction bec in addition the bag had some big spots she hadn't mentioned. I like the bag and wasn't willing to pay a lot for shipping on top (bec you should send insured etc).
if the bag is authentic like you say and you want to keep it they deserve their money - although I know what weird stuff can happen. i will never understand these sellers that do not respond to a paypal claim - so first of all this isn't your fault really. even if they are upset, it is not like you intentionally tried to take both.
did you file about non-authenticity of the bag? maybe it is fake after all? to me it sounds like they don't have the card. if it is about re-selling - send the bag back. or offer a reduced price bec the card doesn't match the bag - and accept that you might not sell it easily. but keeping both, unless the bag is really a fake, wouldn't be right IMO.