Shipping calculator now considers item formats!

It's been a long time in coming, but I finally modified the shipping calculator to pay attention to item formats when calculating the shipping cost. The biggest issue has always been a single LP/12", which was previously considered no different from a single CD. My regular customers know that we can't ship an LP for $1.50, but of course the shipping calculator said we could, and after a new customer rightfully complained that he was charged more than what had been calculated for him during checkout, I was moved to fix it once and for all.

I haven't tested all combinations, and I'm sure I'll be modifying it as time goes on, but at least it gets the single LP right!