Ben, are you using official looking work order forms?

i.e. The kind that larger shops use with all the legal stuff in fine
print that essentially says "if you don't pay we have a right to take
ownership of your car and sell it to pay your bill".

Have a printer who makes forms for auto shops make some up for you,
and ALWAYS have the customer SIGN the form before starting any large
project.

That way you are covered in the event of a customer with no
money....assuming their vehicle is worth more than the work you did...

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And never ever, release the vehicle until the customer pays in
full....they can ride the bus until they come up with the cash.