When filling up at the pump, some Long Islanders may not be getting what they are paying for. Authorities say Island Gas in Valley Stream has been scamming customers by serving up regular gas out of the premium gas pump. They say the station then keeps the difference. Investigators say USA Gas in Franklin Square and Valley Stream were doing the same thing. Nassau County Consumer Affairs Commissioner Roger Bogsted says the stations were selling between 20,000 and 50,000 gallons a month. The owners of all three stations face fines of $10,000 from consumer affairs. They are also being charged by the district attorney, and may be hit with an additional $25,000 in fines.