Multiple sizes in photo book order in iPhoto

I recently made photo books in iPhoto after my daughters wedding. I wanted a mix of sizes, although the books were all exactly the same layout. I couldn't find a way to put them all one one order so had to make 3 separate orders and pay 3 lots of postage, is there a way to add other items or will the system only support one type/size of product per order?

The books arrived promptly and I am very pleased with them, but I would have preferred not to have to pay 3 separate lots of postage.