Link to the lodgement receipt on Aust Post website so you can print off your own, or just have a peek at what they look like:

http://auspost.com.au/media/documents/registered-post-lodgement-receipt.pdf

I use the lodgement receipt with all my pre-paid articles including express post and pre-paid satchels - I just cross off one of the options and write in the product I am using, complete addressee details etc and attach the tracking number to form.

I have never had a problem having it stamped when I have lodged items at several post offices, and it also saves having to match receipts with postcode only to actual parcels sent. 

The completed lodgement receipt with tracking or article number attached provides every thing required to meet paypal requirements when it is date stamped at the post office.