The item is unusually heavy for its price. For example, a ten pound bucket of UltraCal 30 costs less than $20, so the cost of the product doesn't compensate for the extra shipping cost.
The item contains dangerous substances. Polyurethanes contain isocynates which can't be shipped by air. Spray cans contain compressed propellants and can't be shipped by air. Usually items marked with an "S" or have hte flame icon can't be shipped by UPS air or US Mail.
Round Items, additional handling
Some larger barrels (50 lbs.) of plaster and gypsum are round and therefore fall under UPS's "Additional Handling" rule. These barrels require a $4.00 fee in addition to the regular shipping charge. The US Mail doesn't have this rule.