This article looks at how to set up boxes and set it so that a product goes into one box on its own but then goes into another box when placed in the cart with another item.
How do I configure the boxes for the small items so the system knows that they can fit into the boxes with the other larger items?
Firstly you need to configure both the small and large box’s in Catalog>Managed Shipping Boxes.
The image below shows two boxes that have been configured for both small and large products.
As long as the small box is for the small product’s dimensions this will be used first.
For the large product add the large box only.
When a small product is added to the cart it should only use the small box, but when the large box is in the cart with the small product they both will go in the large box together.