Both Amazon and Newegg sell NUC models with the designation BOXNUC. The BOXNUCs are cheaper and I've found no explanation of the difference. Example: Amazon has both NUC5i5RYH and BOXNUC5i5RYH. The BOXNUC is $80 cheaper.
Amazon.com: Intel Next Unit of Computing 2.5-Inch Drive Option BOXNUC5I3RYH: Computers & Accessories