Bellagina, is it possible your buyer is thinking in terms of testing the waters with claiming items to be fake?
You've probably done the best thing by passing on the link, and there have been some excellent answers already given. I suppose you could contact the buyer again and state you're perfectly prepared to refund the money and have the item sent back... If you word it along these lines, perhaps?
Dear buyer,
I'm so sorry you feel unhappy with the item. I can assure it's a genuine Tupperware piece, and I can of course supply PayPal with the information from Tupperware demonstrating its genuineness. If you do still feel unhappy with your purchase, I have no hesitation in offering you a full refund. Simply return the item to me (you'll need to send it by registered mail) and I will refund the full amount including the return postage. I don't want any of my buyers to feel dissatisfied with the quality of their purchases, and I am sorry you feel in any way dubious about this item which is, as stated, perfectly genuine.
Please let me know if you decide to return this item.
Yours sincerely,
[name]
It's certainly possible that, if your buyer realises you're prepared to back the genuineness of the product but that you have the excellent customer service to offer a refund upon return, she'll realise the dispute is not going to pay off. It sounds to me as though the buyer is trying for a discount after the fact...