The simple answer is nothing. CBUs, or cluster box units, are a product of the USPS, which licenses out manufacturing. Both manufacturers must produce the units with the same materials to the exact specifications, including the texture of the paint. The units are so similar that parts are interchangeable between the two manufacturers. 

 So, why is there a price difference between the Florence and Salsbury CBUs? 

-Florence is the larger of the two manufacturers and is located in Manhattan, Kansas. 

-Salsbury is located in Los Angeles, California. They have a 600,000-square-foot warehouse in the greater Los Angeles area. That is usually all the explanation needed. 

The bottom line is that both manufacturers produce the same product using the same materials and have the same shipping lead times. If you order one CBU from each manufacturer, the only difference you will see is the price you pay for each unit.