When I started in this business, I did everything with a handshake and smile. Until I got burned. JUST ONCE was all it took for me. I write up a contract for regular accounts, or for one time jobs, I use a carbon copy work order, with a signature. That one work order has insured my getting paid for a job more than once. I had a General Manager that wasn't happy with my price AFTER I had done the work that the Service Manager authorized. The only reason I got the price I originally quote was because I had the Serice Manager's signature. They tried to convince me to change he price after I had done the work because he thought it was too high.
From then on out I NEVER work without getting it all in writing. In one form or another.