We tried Boxify( this was recommended by Shopify btw) but we couldn't get over the fact that their monthly charge is based on calculations not actual orders; this means any customer checking your shipping cost will ding you. We estimated we would have to pay an additional $250+ a month for this app; not a huge cost for a business but it's the disappointment to find how we are yet getting nickeled and dimed.
For anyone else considering another option:
For our oversized packages, we decided to use the dimensional weights for each items instead of their true weight. There are many websites online that will take the dimensions of a package and then return its dimensional weight that UPS, FedEx, and USPS use to calculate a package. with this setup our shipping is much closer to the actual cost now. Still, this method isn't prefect since there is the issue that Shopify forces your site to using just one DEFAULT box. Try using a default box that isn't too small if you don't want your USPS giving you rates that would go into a flat rate box.
@sfDisplayYet another work around.... The problem with this is that you are entering fictitious product data (dimensional weight) instead of actual data (LxBxH & weight) into your backend. That will sync a 3rd party stock control system if you use one, and then all your data is fictitious. If you ever change to real data, then you need to review every single product! If you use another sales platform that DOES use real product data then you are stuck because you can only have on or the other in your system.
Shopify... is anyone listening?