We've been wondering what would happen with shipping prices from Book Depository ever since Amazon purchased it. In a related development, second-hand book seller AbeBooks (which is also owned by Amazon) has begun offering free Australian shipping on some titles.
AbeBooks is one of my favourite book shopping options, but I'd failed to notice the "free Australian shipping" option before. A quick search suggests that it covers a range of sellers, not just those based in Australia. Free shipping won't always make a second-hand book cheaper — I'd be double-checking to see if you could get the same title with postage for less via someone else — but it's a nice option to have.