Assigned Mix Box

An option to assign boxes to mixed boxes is available by checking these two check boxes in System Control, under the shipping tab:

And

The inventory item needs to be setup as a mixed box:

And then a box needs to be assigned under the Mixed Box Tab:

Create a Sales Order and under the Quick Entry Tab and then select your Mixed Box from the dropdown

Then add the Quantity and Price and then click Add to New Box

This will add the Mixed Box to the Sales order