If the item is going less than 50km (I think) postage is cheap, otherwise C&S tends to be at least as cheap as any other postage method.

If your items are heavy but not too bulky, you could look at the AP boxes - they all have a limit of 15kg through C&S.

eg a Bx1 (22 x 16 x 7.7) costs $1.75 plus $6.50 postage. For over 500g significantly cheaper than a 3kg satchel. Unfortunately a full size Hardcover won't fit in one. :_|

A Bx4 is under $17 p+p and is 43 x 32 x 14.