可以在PayPal的Help Center网上进行：
Can I cancel a payment that I've already sent?
Answer :
You can cancel only unclaimed payments. If the payment is complete, you can contact the person who received the payment and request a refund.
To cancel an unclaimed payment:
1.Log in to your PayPal account.
2.Click History.
3.Click Cancel under Order Status/Actions.
4.Click Cancel Payment next to the unclaimed payment.
Note:
A payment is automatically cancelled if the recipient doesn’t accept it within 30 days.
If you paid with a credit card, it can take up to 30 days for a refund to appear on your card statement.
If you want to request a refund for a completed payment, here’s how to find a seller’s contact information:
1.Log in to your PayPal account.
2.Click History.
3.Click Details next to the transaction.
