Boxt has two options for packing case-packed shipments:

Option 1 is called Bulk Edit and is similar to the Seller Central case-packed view. Only 1 ASIN/MSKU is allowed in each box. For example, 20 cases of ASIN #1 may be placed in Boxes 1, 2, 3, and 4. ASIN #2 would go into boxes 5, 6, 7, and 8.

Option 2 is called Pack Mixed SKU Boxes which is a button in the lower right corner of the Bulk Edit view. This puts the shipment in regular Boxt mode allowing you to pack any ASIN/SKU in any box using the case-packed quantities. 

It will warn the user that they will not be able to switch back to Bulk Edit view but that is generally fine.