Has there been a problem in the past by leaving feedback upon payment? I usually just leave it once I've posted but usually send a message after they've paid as a thankyou, advising them of postage and to let me know if there are any problems when it arrives.

Rarely (from my limited selling experience) will you find someone who is dodgy, so leaving feedback right away is a good/positive way to start the transaction, as someone once said on the boards.