Registered Post has been replaced by Signature On Delivery (SOD), but is essentially much the same thing with a different name. The service costs an additional $2.95 and includes insurance cover of up to $100, with extra cover available (at an extra cost, of course).

Returning the item with on-line tracking (SOD) is actually for your benefit, not the seller's, so he/she can't tell Paypal that the item never arrived and must have 'gone astray in the mail', so don't be tempted to save $2.95 by not including SOD.