The app I use includes some basic contracts, that you can edit or adjust as needed. The option is there to for you and the customer to each sign and then you ask them to send it back to you before you do the job.
I also write in (most jobs) that payment is due immediately upon the completion of the job. !
I would like to suggest a wonderful app that we use in our business. It’s made for sole proprietors or small contractor businesses.
(Am I allowed to name it on this website?)
It is called JOIST! It’s the easiest, quickest, simplest estimate and invoicing program that you use on your iPhone . There’s a free version and a paid version which you can link to your QuickBooks. My wife , (Who had no experience at all in the office management) does ALL of our business with it!
hope this helps some !