Kim:
At the moment the problems is - What happens when you have a "flat-rate" and a USPS item on the same order or even 2 "flat-rates"?
I am just now dealing with this - basically we sell two types of items: Toys, which can be big and bulky, and workbooks, which qualify for Media Mail, very cheap and easy. I want to stay competitive by offering low rates for workbooks - starting table rate of $4.50 for one item - but, if they order a doll-house, and chose the table rate - boom, I'm eating $20. My initial solution would be a default choice: Weight (class 1) + Table (class 2) = Weight (class 1). If there are no class 1 items then table rate applies. If there are any class 1 items, then it defaults to weight. At this point, I'm just putting a note that a shipping surcharge may apply. A poor way to do it.