Build a professional invoice from the items you want to charge, then send it before you leave. Fixington is an invoicing app for people who would rather do the paperwork while the job is fresh.
Example
Add the lines you want to charge. Labour, materials, a call-out, a variation. The invoice uses your wording rather than a fixed template you have to fight.
It is the same job you quoted. You are not retyping the customer, and you are not starting from a blank document in another app.
A few taps, then hand the invoice to email, iMessage, or any other app you use with that customer. If they reply in the thread you already have, the invoice is already in it.
On a subscription, the web app at app.fixington.com can also download the invoice, which is the copy you want when you are at a desk or sending it on again later.
Write the invoice on the phone at the job, or on an iPad in the evening. Subscribers can open the same invoice in the browser. A new phone logs back into the account and the invoices are still there.
The invoice stays separate from the quote, so you can see what you estimated and what you actually charged.