It’s reasonable for a plumber to require a deposit in advance for large jobs, or in pre-arranged payments as the job progresses.
But, always reserve a percentage of the total cost until the work is completed as promised.
It’s important to know, at the end of the job, that everything was done.
To make sure you’ve received what was promised, ask the plumber to give you a detailed estimate before the project begins, detailing the work to be done, parts needed, a time line and projected completion date.
At the end of the finished project, before you make the final payment, ask for a detailed invoice.
The final invoice, along with the first estimate, will take you through the job, step by step, so you’ll know what was done and what was installed.
Only if everything meets with your satisfaction, and there are no problems to be addressed, should you make the final payment.