Elevation Charges

Assume you have rates that charge a different amount per gallon depending on the elevation. For example, customers on rate X who live between sea level and 500 feet are charged $0.001 per gallon; customers living over 500 feet are charged $0.0015 per gallon. Customers on rate Y who live between sea level and 500 feet are charged $0.002 per gallon; over 500 feet are charged $0.0025.