Was it an auction or a buy it now? If it was a buy it now, you could contact the seller and tell him you made a mistake, he might refuse the paypal payment for you..
If it was an auction, i doubt they'll do it for you..
Ask him/her, ya never know, ive had one or two cancelled, just mail them through ebay, a lot of sellers dont want the hassle, so will just null your bid..