I've spent hours researching this and wanted to confirm with this community what I'm seeing.
As a background our shop (www.FluffyWeight.com) sells weighted stuffed animals. they are physically big and heavy so shipping is a large expense which we pass on to consumers in checkout. We use Shopify Shipping to get the rates.
I'm trying to find a solution to accurately show the rates for shipping the different configurations items (two different sizes) and number of items. We have set boxes for different combinations.
With Shopify's setup we can only have the one default package that is used for a single large item. This overcharges when we ship a mini or undercharges when we ship multiple items. A well known issue that is often addressed with apps.
However, I've looked at Boxify and others but they can't pull in Shopify's rates and the difference from the standard UPS rates is significant for our big items. People won't pay those.
My best solution is to setup a bunch of rates by rough shipping zones and by weights. It's a lot of manual work but it is free. I haven't been able to figure out another options, but wanted to check with you all to see if I'm missing something.