Questions about default packaging size and shipping rates?

I figured out a way around this which requires background knowledge of your product weights and package sizes. Don’t bother with carrier-calculated rates, just bypass them and do everything with custom rates only. Yes it’s more work, but gives the control needed for the seller to more accurately charge the correct rates. It’s not a perfect solution, nor is it an easy one as it takes a lot of extra work, but I think it’s better than using calculated rates with only a single box size possible (thanks Shopify).