I have the same issue.
I'm running 2.65. I'm using USPS API, and I think nopCommerce is by default assuming each line item will be shipped in a separate package (or is adding the dimensions together improperly) instead of adding the dimensions and weights together for each line item to get one dimensional weight for the shipment.
For example, I have a product with shipping dimensions 10" x 8" x 1", 0.6 lbs. I'm adding $4.00 per line item and $1.95 per order for addl. handling. If I add just 1 item to my cart, the shipping cost for USPS Priority Mail is calculated at approx. $12.00, which is correct, approx. $6.00 shipping cost + $6.00 addl. handling.
But if I add 3 of these items to my cart, the shipping calc. comes back at approx. $44, which includes $14 for addl. handling, thus approx. $30 in actual postage. This is not correct since three of these items would be 10" x 8" x 3" and 1.8 lbs., and postage just slightly more than shipping just 1 item.
Does a more recent version of nopCommerce fix this issue?
Thanks for any insight you have!