OK, I spent some time on the USPS website and a few others. In zones 5-8 If the dimensional weight calculation is greater than the actual weight, you are billed at the dimensional weight unless it is a light item then it's called ballon rate and is considered 20lbs.
Here is another link titled "Save Time by Doing it Right"
[
www.usps.com]
DIM weight is determined by multiplying a package's length, width, and height and applying a volumetric divisor.1 DIM weight = (L X W X H)/194.
The bottom section of "Don't Get Boxed In" has info on zones 1-4 as well as 5-8 info.
[
www.usps.com]
I found a few different numbers from various sites about zones 1-4 but they all were a calculation, not 12x12 max. one saying USPS does not charge for DIM weight when shipping to zones 1-4 unless the length
+ girth of the package exceeds 84”. Girth is measured by (2x width) + (2x height).
[
www.efulfillmentservice.com]
I'm not sure this info helps answers the question but thought I'd share.