A wholesaler of custom-built air-conditioning units for commercial buildings gathered the following monthly data relating to units shipped and total shipping expense: Month Unita shipped Total shipping expense January 2 $2,000 February 5 $2,500 March 3 $1,900 April 4 $2,200 May 6 $2,500 1 June 7 $3,120 July 1 $1,400 Using the high-low method estimate the cost formula for shipping expenses, calculate the variable shipping expense as well as fixed shipping expense per unit.