If you have proof of shipment you should be OK, I think paypal want some sort of proof that shows a delivery postcode and that the item was lodged.

I have won a case before with no proof whatsoever, but I was still charged the $15 chargeback fee. I think this fee is instant as soon as a chargeback is intiated, I always thought it was for if you want paypal to fight it but I don't think so anymore. I have less than 1 per year out of a lot of transactions.

As far as the banks are concerned are they calling all forms of chargeback an  "unathorised transaction"? I always assumed this was if someone (the buyer) used a card without authorisation, in which case they are guilty of fraud and should be reported to the police and ebay. Maybe banks class it as unauthorised if one of their customers uses a card to access money they don;t have or something.

If they have purchased from other sellers I usually contact them as well.