When you have chosen to add a proposed FBA shipment into an already existing working Shipment, the items that you have added will still need box assignments for their Box Level Content.
To make sure the items that were added are assigned to boxes, locate the shipment the items were added to under Working Shipments, click the 3 horizontal lines to the right of it and select Box Content Information.
This will bring you to a screen similar to the Box Content page you were on when you initially completed the original box assignments for the shipment. You will see that you have your previously assigned items there, as well as your new items that still require an assignment in gray:
To assign them to a box, click the arrow next to Assign and choose the box to put the items in, and the quantity you’d like in that box.
Repeat these steps until all of the items in the shipment have been assigned to boxes.
For in-depth steps to assign box content, please see Providing Box Content - Private Workflow
Resending Your Box Content Information
Once all of your changes have been made and you are happy with your box assignments, the final step is to click the Transmit Updated Boxes button in the lower right corner of the page.
This will resend the feed data for your contents over to Seller Central, and you will see the Box Weight and Dimensions window once more to match the adjustments made to the new box assignments.
If adjustments need to be made to existing boxes, or if you have added a box to the previously sent Box Contents, click in the applicable fields to update this information.
Once the weight & dimensions for all the boxes is correct, click Submit All.
Or, if this step was skipped previously, click Skip This Step to bypass it once more so you can made the adjustments in Seller Central.
Note: If you have already calculated and paid for label charges in Seller Central, you will want to void the charges, recalculate your charges and reprint your shipping labels with the correct information.