You need to make sure that the buyer is aware of the risk and also include a recommendation that registered post/insurance is a more secure option.   No you don't actually. There is no requirement I am aware of that enforces a seller to inform a buyer of risk assessment strategies, it's up to the buyer to accept the postage/delivery method offered or request an alternative prior to purchase.

http://www.austlii.edu.au/au/legis/nsw/consol_act/soga1923128/s35.html

(2) Unless otherwise authorised by the buyer,

the seller must make such contract with the carrier on behalf of the buyer as may be reasonable, having regard to the nature of the goods and the other circumstances of the case.

If the seller omit so to do, and the goods are lost or damaged in course of transit, the buyer may decline to treat the delivery to the carrier as a delivery to the buyer, or may hold the seller responsible in damages.

The highlighted piece of the SOGA clause above is the reason (and the legal requirement if you are using deemed delivery) why sellers need to include the warning in their terms and conditions. By offering registered post, stating the risk, the point at which the items title changes hands and allowing the buyer to choose then the buyer has AUTHORIZED  that mode of freight to be used by the seller