Simple Plugin to Calculate Pricing

I have built a site for a heating Oil Company and they want a quick quote system so that customers can find out how much it will cost them to refil their heating Oil. They have two products kerosene and Gas Oil and the price varies depending on how many litres you order. Can anyone suggest a simple plugin or solution to this issue. They do not want to sell via their website just offer the customer a quote and perhaps take a record of each quote given.