Lets say you bake cupcakes for extra income. You need to buy supplies etc to do it. So you only keep a small portion of the actual selling price of the cupcake. Lets say you will deliver 100 cupcakes for $100. So a customer of yours says "we would really like to have more cupcakes but didn't budget for it, so we want you you to deliver 10 cupcakes for $10". Could you do it? Would it make any economic sense to burn the same amount of fuel in your car and use the same of time to deliver 10? Remember just like the heating oil company you actually are keeping only a small portion of the selling price since you have to pay for supplies and use your vehicle to deliver the product.