Your Car Breaks Down and You Are Stranded on a Highway

Your car breaks down and you are stranded on a highway. You call a tow truck which takes your car to a garage and replaces a faulty part. You do not have enough cash to pay, so you give the owner your address to send the bill for towing and repairs, and venture on your way. When the bill arrives, you find the prices are high. After consulting another garage your suspicions are confirmed. The mechanic sends you a notice threatening legal action and you decide to defend it using certain legal principles. What principles will you use for your defence, and what will be the probable outcome?