The reasonable lost rate is 0%..... 2 or 3% should be considered a 'you've been scammed' rate, not a 'lost' rate.
The safest way to avoid any lost rates is to just get tracking, or record an article I.D. on $7.20 postage. There are too many dishonest people out there more than willing to take advantage when after receiving their item, they KNOW their package has not, and cannot be tracked.
It is extremely rare for items to actually get lost within Australia, and if they do not get returned to sender, it is more than likely a scam by the buyer to get something for free.
Odd how only untraceable items get 'lost' isn't it???
If their address is correct, and the letter hasn't been returned to sender, I'd consider the buyer a liar, and block them from future purchases. Personally, I'd rather not do that, so it is easier to just get tracking or an article ID at least.

There are a lot of honest people out there, but at the end of the day there are a lot of dishonest people out there too which you need to protect yourself against.