This form certifies the purchase price and determines the sales tax you must pay when registering the vehicle. Pay vehicle sales tax, prove the sales tax was paid or prove that a vehicle registration is exempt from sales tax.I'd like to take this time to review the sample contract with you to help you understand the ins and outs of a purchase contract. Refinance your existing auto loan for better rates. Suffolk Credit Union offers a variety of auto loan calculators like the Car Lease vs. Buy Calculator and Auto Refinance Calculator. I'm new to car financing. What got me confused is that my bank wants purchase agreement or bill of sale before sending out a check to the dealer. Welcome to the Suffolk County Clerk's Office. At one time or another either directly or indirectly, every resident of Suffolk County deals with our office.