You will be able to view which container \ custom box is used for packing by turning on the shipping debug information in the admin area.
HOW DO I TURN DEBUG ON?
Once you have turned on debug, add a product to cart, go to the cart, enter the information required to obtain a shipping price and the debug information will be available.
For some shipping services where the shipping calculation is required on the checkout page (Fastway, Interparcel & Hunter Express) you will need to, add a product to cart, go to the checkout, enter the information required to obtain a shipping price and the debug information will be available.
The debug information looks like a long page of code.
Scroll through the code and find the information about 'Box Packing' Find best container and you will find:
1, Which box was selected for the shipment.
2, How many boxes were selected for the shipment. There may be more than one box and the boxes may be different sizes.
Find Best Container ( Mailing box, Bx 2 ) for items 9,
Prepare 6 items to pack
Find best container ( Mailing box, Bx 5 ) for items 88, 88, 88, 88, 88, 9,