Well if it helps I can tell you that I got a big box back in May with 10 carded deluxes, a boxed deluxe and a boxed Universe Unicron, all in the one larger box from the US.
So I can say with fair certainty that 10 carded basics and 4 carded deluxes should fit in the same size box.
The seller sent it via the USPS (cant recall if it was Priority or Express though) and Australia Post had no problem with that one at all.

From what I could see Australia Post doesnt seem to actually have anything on their site that mentions restrictions of size/weight on incoming packages from overseas, only outgoing ones, so I'm not sure what else to say on the matter.