Re: USPS flat rate and dimesions
We use the addon mentioned by stevesh and it works very well for us in filtering items by volume. Doing that removes the smaller flatrate boxes from the choices presented to the customer when the items added to the cart exceed the volume of the box. It includes volumes for small, med, large, regional a and regional b boxes.
It does require a bit of work and tweaking of your individual product weight/volume. The addon requires a sql modification (included with the addon) to allow the 6th-11th digits to the right of the decimal weight to be used for product volume in cubic inches.
This module HAS NOT been updated to include the Jan 2012 USPS changes but those changes mainly affect existing International first class packages and the newly added products.
In your example, FlatRate does not mean USPS flatrate. It means that you will ship any package for a flatrate of $xx.xx regardless of size.
Rick
RixStix (dot) com
aka: ChainWeavers (dot) com