This page's plumbing estimate proposes four lines: a service call or diagnostic, labor, parts and materials, and a permit fee. Pricing them separately before the job starts means the client is agreeing to a real breakdown, not a single guessed number — if the diagnostic turns up more work than expected, you can point to exactly which line changed rather than re-explaining the whole price.
Service call and diagnostic time get quoted as their own line even on jobs you're confident will need a repair, since it's billable whether or not the client goes ahead with anything beyond it. Labor is priced by the hour rather than a flat job rate, parts are estimated on their own line rather than folded into labor, and a permit fee is only worth including if the scope actually requires pulling one.
Put your license number as a note under the business block on the estimate itself, not just the invoice that follows it — a client comparing quotes from more than one plumber will look for it. A custom field for the job address is worth adding for any property you don't own, so the estimate is unambiguous about which address the price applies to.
Once the client accepts the price, convert this estimate into an invoice in one step — the lines, the numbering, and the design carry over, so nothing has to be retyped. Start from the plumbing invoice template directly if you're billing a job you didn't quote first, and once the invoice is paid, record the payment and issue a receipt for it the same way.
An estimate doesn't need a due date the way an invoice does, but it's worth noting how long the price is good for and what payment terms will apply once it converts — due on receipt for a quick service call, Net 15 or Net 30 for a bigger job — so the client isn't surprised by the terms on the invoice that follows.