Apart from which, the 5kg express post satchel is a 'Paypal approved' method of posting so the seller is covered and any attempt at a dodgy claim would be seen for that on the spot.
The 5kg express post satchel costs $18.50 and includes proof of postage as long as it's posted over the counter at a post office. The info on the Auspost site specifically sets out the Paypal/eBay side of things too:
Express Post 5kg satchel I'm not excusing the pocketing of the difference in the postage cost versus what was charged, just thinking through why the satchel may have been used. But then, I would've packaged the box set and happily refunded the postage difference...