Well things don't go exactly like that at many gas stations. What actually happens is the big companies pump gas from the big trucks into your big tanks under your average gas station. This amount of gasoline dumped is not charged up front but at the end of each month. The only thing that is reported is how many thousands of gallons are pumped each time. As you know, gas prices are constantly changing and what the big companies do is call you (gas station manager) when the prices do change and tell you to change your prices according to theirs (usually about 5-10 cents more per gallon). The companies know the amount of gallons sold for whatever amount they sold it to you for and keep all that on record until the end of the month. At the end of the month the gas company sends you a bill for however much gas you sold so far during that time. Every ounce of gas is recorded and yes gas companies are making a killing but at many times the gas stations are not.