eBay has been promising cheaper postage options for heavy users as part of its partnership with Australia Post for quite a while now, and it has finally delivered with three flat-rate postage boxes which can be used to send items weighing up to 15kg.
There are three box sizes on offer: small, medium and large. With each, you purchase the box itself from any Australia Post outlet, and then purchase and add a pre-paid postage label via the Click & Send online service. That means you won't necessarily have to queue to pay for postage, though only the smallest box will actually fit in a street posting box -- for the larger items, you'll still need to lodge over the counter. (That said, using Click & Send is still handy since you can then track your item online.)
The small box (220mm by 160mm by 77mm) costs $1.70 plus $6.50 for postage ($8.20 total). The medium (310mm by 225mm by 102mm) is $2.25 plus $10.50 ($12.75). The large (430mm by 305mm by 140mm) is $3.00 plus $13.25 ($16.25).
There's a discount for purchasing multiple boxes, which is pretty substantial if you buy a lot (the price of the small box falls from $1.70 to $0.58 if you buy more than 200). However, there's no postage discount for sending multiple items.
It won't suit everyone, but it does provide a no-argument way of pricing postage for larger items. For smaller goods (such as a single DVD), you may still be better off with Express Post: that costs $6.40 for a single B4 satchel, which is cheaper than the postage on the smallest flat-rate box.