Sticky labels would be ideal, then you can just stick them on the packages - however, I never use them. I just print a despatch note from the PayPal transaction, then go to the PO myself and buy stamps for the item, and make sure to get Proof of Postage in case of no-delivery. You have to go to the PO anyway, so unless you are mailing loads of things, I wouldn't want to pre-label. In any case, it has been known for me to be somewhat "out" in calculating postage so I'd rather be sure that I am paying the right about, by letting the PO weigh and stamp it for me.