It weighs just over 50 pounds, so it could go UPS. My take is that Markham has a deal with the trucking co to give them the business in exchange for 2 day delivery promise. That deal probably extends to other, heavier items they sell too.
If this were shipped via UPS, the cost is $72.00 with 3 day delivery. It would be as low as $32 if it took the slow boat.