Go to Paypal.
Make sure the address you gave Paypal is correct, including your apartment number or PO box number , if any.
Find your original payment.
You will see the seller's email address. This is also her PP address.
Copy the address
Go to the Send Money service.
Paste the address (email) into the Send Money form.
Add the postage amount. Make sure you have the right currency.
In the text box put a note about the transaction, including the transaction number which was on the original payment. (This is not absolutely necessary, but will be VERY helpful, especially if the seller has a lot of transactions.)
Send.
Ta-- daaah!