From the email:
Simply place an order before midnight, Monday the 11th of May (wherever you are in Australia) with the promotion code FREEBIE and you will receive free shipping on your order. The promotion code can be used as many times as you, or your family and friends, want on any orders between now and then.
The promotion code field where you enter the word FREEBIE is just above the section where you choose your payment options like Credit Card, PayPal, Bank Transfer, etc. This is on the last page of the checkout just before you complete your order.
Please note: You may need to click the word "Apply" next to the promotion code field to receive your free shipping discount if it has not automatically applied it.
This offer is only available to Australian shipping addresses. If you are overseas and want to ship to an Australian address then you can use this promotional code too.
It is not applicable for Australia Post Express Post OR eBooks as the discount code applies only to the shipping cost of a standard Australia Post parcel and not to the overall order.
Free shipping has been available for a while, if you clicked from Booko through to Booktopia.
Still, a good deal, given how much postage costs in Australia.