384 cubic inches ÷ 139 = 2.76, rounded up to 3 lb for UPS Daily Rates and FedEx. USPS bills actual weight because the box is not larger than 1 cubic foot.
How heavy does a 12x8x4 box need to be to avoid DIM charges?
With UPS and FedEx, anything packed under 3 lb is billed as 3 lb. Above that, you pay actual weight.