Amazon does not require you to provide Box Level Content for shipments that only have 1 box. It is recommended, if you are already aware you will be doing a single box shipment, to leave the Box Content feature off in InventoryLab.
We know that it's not always possible to know in advance if everything will fit in a box, so we would like to explain a little more about what happens when you choose to turn on Box Content in InventoryLab, the warning you will get in Seller Central if you only have one box, and most importantly, that it's ok to complete your shipment still. :)
Turning the Box Content feature to "On" in InventoryLab alerts Amazon that you intend to send your box content information over to them in something called a feed. At that time, Amazon's systems select "Multiple boxes" and "Amazon Marketplace Web Services" automatically and disable the ability to alter them. This is because they assume you are sending multiple boxes (since box level detail is not needed on single box shipments) and the feed previous mentioned is received by them through that Amazon Marketplace Web Services API.
If you are not sure if your shipment will contain more than one box, that is ok. However, you should be aware that when you go to select the number of boxes you have in Amazon, you will see a little warning asking you to change the dropdown box to "Everything in one box".
While there is not a way to change this setting, since Amazon has greyed it out automatically, it does not stop your ability to calculate your costs, print your label, and create/complete your shipment. In the long run, you are simply providing Amazon with just a little more detail than they need for that particular shipment. :)