Can you change the declared value of my order?

No since this is customs fraud. Doing this also does not protect you from later charges by Australian Customs. They may send you a bill anytime in the next five years if your order was not accurately declare. While there are some companies out there that will do this for you, it is worth considering if you want to use the services of someone that is willing to commit fraud.

It is worth remembering that any order under A$1000 is GST and duty free. Accurately declaring the item's value also means that you have the correct insurance value for your items in case anything goes wrong.