i did this for a friend of mine. she doesn't have a paypal account and i bought a bag for her - i told the seller to ship it directly to her. she didn't want to because it was not confirmed but paypal told her that all i needed to do was add the other address to my paypal account and that way she would be covered.
For seller protection, you can only ship to the address shown on the PP transaction page. If the buyer wants it shipped elsewhere, she has to ADD the other address to her PP account, and then likely re-pay and specify the new shipping address.