Well, you know wrong. BookDepository does not do free delivery; what it does instead is inflate the book prices it quotes you depending on the country you’re from. Given they’re a British company, you can expect the prices on offer to British users will be somewhat lower than those on offer to Aussies (the titles I checked were by around $3 cheaper).
The funny thing about it is that the discount has nothing to do with the country the books will be shipped to and everything to do with the country you’re using the BookDepository website from. For example, were my English mother in law to buy me a book and have it sent over to me at Melbourne, she will be paying less than it would cost me to order the same book and have it delivered to the same Melbourne address. It goes the other way around: if I was to buy my mother in law a book from Australia, it would cost more than it would cost her to order herself the same book.
Only that two can play in that game. If you can make yourself appear British before BookDepository’s eyes, you would also be able to enjoy the same rates offered to British users. There are ways of giving BookDepository the illusion you are British, one of which is using the services of a British VPN server to acquire a British IP address before the eyes of external lookers. It is important to note there is nothing illegal about doing so; you will only be using/abusing BookDepository’s rather eccentric mechanism for determining delivery fees. No one is preventing BookDepository from charging delivery fees that depend on shipments' destinations (the way its new owner, Amazon, does); they're doing what they're doing out of the assumption their claim of offering "free shipping" would attract extra clientele.
The VPN services through which you can acquire these postage discounts will usually cost you money, but if you order enough books you can get your money back and more. As far as I am concerned, this VPN trick is a fine example for the way opening one's business to online scrutiny exposes all sorts of issues in business models that were designed by some ingenious marketing team.
Image by spike55151, Creative Commons license