By signing this contract, I choose to purchase the motor vehicle on credit according to the terms of this contract. Bill of sale contracts are issued to fill out and get a vehicle to.This document terms as the source for defining all the terms and conditions that are required while setting up a sale and purchase of a car. A car sale payment agreement is used when the purchase price of the car is paid in instalments. 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.