I've been puzzled by this as well.
I understand there are a large number of third party sellers on amazon, operating as businesses etc and I've noticed they almost exclusively don't ship international especially to Australia.
Then there is items sold by Amazon LLC which I understand is Amazon themselves and it's really hit and miss what can be shipped.
I randomly put in my shopping cart all the Star Wars UCS sets but it would only ship the SSD and Death Star.
The same goes for any of the modular buildings.
Surely the death star box the bigger than all of them and they were charging 70 bucks for postage which I wouldn't have a problem with anyways.
Incidentally the postage for the SSD is 35 bucks.
Being in Australia the retailers here have been complaining endlessly how shoppers buy stuff overseas now the AUD is so high and it wouldn't surprise me if certain items have been selectively excluded from being shipped here to protect the local retailers. It has happened with some clothing brands.
So much for the global economy.