I have a store that I'm selling books. I'm shipping via USPS Priority Flat Rate. I can fit different quantities of books in the different boxes (1 book in Flat Rate Envelope, 5 books in Medium Flat Rate Box). I don't know how to set up the shipping options so that, depending on the number of books a customer orders, the correct shipping price corresponding to the correct size box will be used. I've been losing sleep over this. All help is greatly appreciated.