All items in your cart, get added to 1 box
Once you add this box to your cart - you can shop the site, and add other items.
Any additional items that you add to your shopping cart (even another box) - gets combined into one box. This is a benefit, as you can shop the site and add additional items to your box, that may not have been suggested during the Build-A-Gift step by step.
So feel free to shop away on the site and add additional items to your box. (Just remember they won't get separated into different unique boxes per shipment)