Wow it's been a while posting here since my WoW days :P
Ok so I've been selling a lot of these Motif books to make gold for the mount. It seems the calculations for postage are done in an odd manner. See pictures below:
One book for 1000g COD has a cost of 110g postage.
Two books for 2000g COD have a cost of 410g postage.
So if I were to send the two books individually it would cost me 220g instead of 410g. Basically sending things individually reduces your costs by a large margin!
Hope this helps.