Hi @PomandChi ,
Shopify only offers one box size to be sent off for live rate calculations, regardless of the size of the actual order. The problem with using Shopify’s single box size is that you will be undercharging for some orders.
To handle this well, along with the apps @Kimi_2 mentioned, you can also use the Multi Carrier Shipping Label app, which helps you handle this with ease. With the box packing method in the app, you can set up different packages for your products based on their weight & dimensions.
The app automatically chooses the right box and calculates the rates accordingly. Also, with the app, you can print labels, and also automate the tracking process.