After many hours of testing.
We have established that two shipping services at USPS (Priority Mail and Priority Mail Express) will use Dimensional weight if:
- One of the product unit dimensions (L x W X H) are over 12"'s in length, this is considered a large package.
- Your product dimensional weight is greater than the actual weight.
- The distance between the origin Zip Code and the destination Zip Code fall into zones 8-9
Zones are established starting from your Origin Zip Code.
- zone 1 Non-local zones within a 50 mile radius of the point of origination
- zone 2 - 51 to 150 mile radius
- zone 3 - 151 to 300 mile radius
- zone 4 - 301 to 600 mile radius
- zone 5 - 601 to 1000 mile radius
- zone 6 - 1001 to 1400 mile radius
- zone 7 - 1401 to 1800 mile radius
- zone 8 - 1801 miles and over
- zone 9* ZIP Codes Assigned For Exceptional Network Circumstances **
More information can be found on the US Postal Service website.
The USPS API used to determine the shipping rate does not consider this information above and will only use the actual weight of the product to determine the shipping rate.
Another problem is that not all the USPS Shipping Services use dimensional weight, only Priority Mail and Priority Mail Express use dimensional weight and the other services will use the actual weight (dead weight) to determine the shipping rate.
The information sent to the API is the L x W x H and the actual weight (dead weight).
If your product falls into the any of the above criteria's then you will notice a difference between the price offered on your website and the official calculator at http://usps.com for the Priority Mail and Priority Mail Express Services if dimensional weight is greater than the dead weight.
We have tried to implement two separate API requests to cater for the different shipping services, one using the actual weight and the other using dimensional weight but the request fails.
USPS does not supply this information in the post request to the API users and therefore any attempts to replicate this outcome have failed.
EXAMPLE USED AT CALCULATE A PRICE AT USPS.COM
Zip code "from": 60422 Flossmoor IL
Zip code "to": 94177- San Francisco CA
Actual weight / Billable weight: 5.5lbs aka 5lbs 8oz
Dimensional weight: 9lb 10.22oz (4.372 kg)
IN THIS EXAMPLE YOU CAN SEE THAT THE ACTUAL WEIGHT WAS ENTERED AND THE PRICE WAS USING THE DIMENSIONAL WEIGHT BASED ON THE DESTINATION ZIP CODE.
Any annomalies in price using USPS Priority Mail and USPS Priority Mail express (and the above listed scenario's) between the Official USPS Calculate a Price and the price offered on your website will have to be accepted as is.