I've checked the terms of the MyUS shipping rates based on weight. They specify their rates
"excludes oversized shipments and palletized shipments with linear dimensions greater than 80 inches (203 cm).?".
Linear dimensions simply means to add the length, width and depth of the package. So taking Borgeman's assumptions of a 20 inch cube would give 60 inch linear dimensions, well within the 80 inch limit and therefore giving us some leeway on the dimensions.
Then it all comes down to weight. We know that Unicron has a weight of 19 pounds. So if we very conservatively assume a packaged weight of 30 pounds, it gives a shipping cost of US$143 using Fedex Economy. But I'm a member and pay with Amex, which gives me a 30% discount off this rate. Therefore given the current A$ is 0.70, my estimated cost is A$143, way cheaper than BBTS and Shipito that base their charges on volumetric weight.
You can test this for yourself at
https://www.myus.com/pricing/
The only risk I can see is if MyUS changes their policy and moves to volumetric weight at some point before this is released in 2021.