The short answer
Publish the arithmetic. One page, its own URL, that explains what an excess is, shows two worked examples either side of the line, says honestly when not to claim, states whether the customer can choose their own repairer, and ends with a link to the estimate form. It is the page that shifts your mix towards private work.
Why customers get it wrong in both directions
One customer has a £520 repair and a £400 excess. They claim, lose their no-claims, pay most of it anyway, and you get insurer rates on a job that should have been simple.
Another has £3,000 of damage and pays privately out of fear of their premium, then agonises for a fortnight over three quotes.
Neither is being foolish. They have no model of how it works, because nobody buys accident repair often enough to build one. The shop that supplies the model gets the enquiry.
The two examples that do the work
Put real numbers on the page, one either side of the threshold.
"Excess £400, repair £520: claiming saves you £120 and puts a claim on your record for five years. Most people pay us directly."
"Excess £400, repair £3,000: claim."
That is the section people screenshot and send to their partner. It is worth more than any amount of copy about your commitment to quality.
The four other things to cover
What an excess is. The amount they pay before the insurer pays anything, applying whether or not they were at fault, though recoverable from a liable third party. A surprising number of customers believe it only applies when at fault.
What a claim costs beyond the excess. No-claims effect, premium at renewal, the years it sits on the record, and the obligation to declare it. Describe the general shape and hand the specifics to their insurer.
Whether they can choose you. Most customers assume the insurer decides. Depending on the market and the policy they usually have more freedom. State it accurately for where you work, and give them the sentence to use when an insurer pushes its own network.
Your approvals, precisely. Name any manufacturer or insurer approval you actually hold. If you hold none, say what you do hold instead. Never imply one you do not; regulators treat it as a serious misrepresentation and competitors report it.
The wording discipline
Two habits keep you safe. Describe general practice — "most policies", "typically", "your insurer will confirm" — and never state an individual's excess, premium effect or no-claims outcome as fact.
Wire it into the estimate flow
Put the question in the form itself: claiming, paying privately, not sure. "Not sure" will be the biggest group, and it tells the estimator to include the arithmetic in the reply instead of guessing which conversation to have.
Then link the page from beside that question and from the estimate email for anyone who answered "not sure". One line, one link, and a round of back-and-forth disappears.
Why it pays
Private work is the work worth having: no insurer schedule, no third-party approval, no thirty-day terms, and a customer who chose you rather than being sent.
Explaining the decision honestly loses you a handful of small claims you did not want and brings you more of the private jobs. Shops that publish this report the same thing: the two-week silence after an estimate gets shorter, because nobody is stuck on a question they were embarrassed to ask.
Where this sits
Our body shop websites include the claim-or-pay page with the arithmetic, linked from the estimate flow, at a fixed price. The wider trade version is on the automotive page. Tell people when not to claim. It is the most profitable honesty available to a body shop.