I've added the weight of each book to the original listing.
This will allow you to calculate cost of postage and save some emails!
Australia Post provide postage bags for regular mail in three sizes.
Up to 500gm $7.20
Up to 3KG, $11.40
Up to 5KG, $14.40
I could physically fit about 4 smaller books in a 3KG bag and the cost would be $11.40 for postage.
I hope this helps!
Cheers
Chris