Invoice before the keys
The best time to be paid is at pickup, and the best way to be paid at pickup is to have the invoice ready before they arrive. If the invoice is written up that night, you have already chosen to chase it.
Make paying the easy option
Bank details on the invoice, a link they can open on their phone, a reference that ties the payment to the invoice so you do not spend Friday matching deposits. Card payment where you can offer it.
Remind on a schedule, in writing
Due date passes: one reminder with the link. A week later: another, still polite, still with the link. Automating these removes the awkwardness that makes workshops skip them, and it means the tone is the same for every customer.
When it needs a phone call
Two written reminders and silence means something is wrong — a dispute you have not heard about, or a customer in trouble. A phone call finds out which. Go in with the invoice open and the photos of the work handy; most 'I'm not paying that' conversations end when they see the old part.
Head it off at the quote
Most payment trouble is decided before the job starts. A quote the customer approved in writing, with the parts priced and the terms on it, leaves nothing to dispute at pickup. For bigger jobs, ask for a deposit against the parts order and say so on the quote; a customer who has paid a third is a customer who is coming back for the car.
Extras found on the hoist go out as their own approval, so the final invoice never carries a surprise. Surprises are what get argued about, and an argued invoice is a late one.
Do not hold the car hostage by accident
Your right to hold a vehicle until the bill is paid varies by state and by what was agreed, and it sours a customer faster than anything. Use it as a last resort, in writing, with the approved quote to hand — not as a habit at the counter.
Keep the record
Every reminder sent, every payment recorded, on the invoice. If it ever goes further, the trail is the case.