I've been looking for a long time for an extension to solve this problem, but it seems nothing is out there to help me out.
We have simple products that are shipped in different boxes with different weights and different dimensions. These boxes are almost never the same. Most extensions assume that you'd ship in maybe 5 different standard boxes or you would have the possibility to fill in only one box per product with weight and dimensions. It's never possible to assign different boxes with different dimensions and weights to one single product.
Have you spoken to https://shipperhq.com/? They're known to be one the most knowledge on this area and have a feature rich SaaS solution. (Born out of Webshopapps).