Calculated shipping not working with orders of multiple items

[pokupoku]

I sell an item that fits into a 16x16x16 box, most just buy one but sometimes people buy 2 or 3. But when someone buys 2 or 3, they are only charged for shipping once and I am only allowed to print one shipping label. How can I fix this? Or is there a way so I can limit only one of these being bought at a time?