here's what it said:

• Proof of posting for the 5kg satchel and Express Post parcels when lodged at an Australia Post retail outlet. This is important if you're a seller using the PayPal online payment gateway to ensure that you qualify for PayPal seller protection.

• Scan events on Express Post satchels and parcels for viewing on our online tracking tool (where available.)^

also wp has a thread about it as well.

http://forums.whirlpool.net.au/archive/1795816

• Correct if you use regular Express Post satchels, however, if you use the Express Post labels then that is acceptable proof for PayPal as it states the name and address of the buyer as well as it has a Post Office date stamp