You sent an invoice.
EBay sent an invoice.
You should send ONE reminder "Your item is packed and ready to go as soon as your payment clears. If you are having problems with Paypal, let me know if I can help." is one I have used from time to time.
I think it is friendly and non-judgemental and has kickstarted the slow payers. The deadbeats still don't respond, so then:
http://resolutioncenter.ebay.ca/