My first hint to you is, to put your code in CODE tags so that your post looks pretty.
My second hint to you is, you know how much fuel each customer purchased right? Then each time there is a new customer add the amount of fuel purchased to a total fuel purchases variable. If you want to get real fancy make a total fuel purchases variable for each type of fuel being sold. If you want to know how how much revenue was incurred, you can either do the same as above, or you can take your three total fuel purchases and multiply them by their respective fuel rate. If you intend to do this inside of an object, you might consider using static variables.
Oh, and fix your post... There is an edit button at the bottom of it...