No information is available for this page. When you buy a car at the dealership, you'll need to sign the car purchase or sale agreement along with other documents.Find out what you'll sign and why. Vehicle Payment Plan Agreement. A vehicle payment plan agreement is a contract between a buyer and seller of a vehicle that agrees to installment payments. 1 If you're financing your purchase ; 3 The actual vehicle purchase agreement ; 4 Buyer's trade in certification ; 5 Warranty disclaimer and documentary fee notice. Once you sign a contract to buy a car, the car is yours. Some dealers will offer a cooling off period as a marketing tool. To draft a legal payment agreement for a vehicle sale, it's essential to include specific details and terms to protect both the buyer and the seller.