Mobile mechanics have the messiest admin in the trades, and it's not close. A garage-based mechanic has a counter, a card machine and a filing cabinet. You have a van, a phone, and a working day that might cover six postcodes, four trips to the motor factors and a customer who wants a receipt while you're still wiping brake cleaner off your hands. If you've been hunting for a mobile mechanic invoicing app, the real requirement isn't features — it's that the whole system has to work from the driver's seat, between jobs, in under a minute. That's a job for messaging, not another app.
Factor receipts: the paper blizzard
Parts are the defining admin load of mobile work. Discs and pads from one factor, a coil pack from another, a battery from wherever had one in stock — each visit generating a thermal receipt that will be unreadable by autumn. Unlike a garage, you're often buying per-job, several times a day, which means the receipts multiply fast and each one belongs to a specific customer's invoice.
Two things go wrong when they aren't captured immediately. First, you under-bill: a £38 sensor bought in a hurry never makes it onto the invoice, and that's £38 straight out of your margin. Second, you under-claim: parts you did pay for never make it into your expenses, so you pay tax on money you spent. The fix is mechanical — photograph every factor receipt at the counter and file it against the vehicle it's for. This is where a WhatsApp-based workflow earns its keep: GraftG, launching soon from Green & Home Ltd, lets you text a photo of the receipt to one number and it's logged. No dashboard, nothing to install, no 'I'll do it tonight'.
Mileage: the biggest expense you're probably not claiming
Here's the number that should get your attention. HMRC's approved mileage rates (AMAP) for cars and vans are 45p per mile for the first 10,000 business miles in a tax year, and 25p per mile after that. A mobile mechanic covering 15,000 business miles a year is sitting on £5,750 of allowable expenses — £4,500 for the first ten thousand miles, £1,250 for the rest — under the simplified mileage method. Miss half of it because you didn't keep a log, and you've overpaid tax on thousands of pounds of expenses you were entitled to.
And mobile mechanics miss more of it than almost anyone, because the driving is the job. Home to first customer, customer to factors, factors back to the same customer, on to the next one — a dozen legs a day, none of them memorable. Reconstructing that in January from a diary and guesswork doesn't survive scrutiny; a log kept on the day does. The workable habit is a one-line message per leg or per day: 'Home to Ashton, 14 miles. Ashton to factors and back, 6.' We've covered the wider case in mileage tracking for UK tradespeople — but no trade has more to gain from it than mobile mechanics. As ever with tax, the rates above are the general rules; check your own situation with an accountant, especially if you're weighing mileage rates against claiming actual vehicle costs, because you generally have to stick with one method for a given vehicle.
Invoice on the driveway, not on the sofa
The golden moment to invoice is before you've left the customer's driveway. The job's fresh, the parts receipts are in your pocket, and the customer is standing right there feeling grateful their car starts. Wait until the evening and three things degrade: you forget the small extras (the bulb, the wiper blade, the extra half hour the seized bolt cost you), the customer's sense of urgency fades, and the invoice joins a backlog you'll resent on Sunday night.
Driveway invoicing needs to be genuinely fast, though — two minutes, phone only. Parts plus labour plus the callout, sent to the customer before you turn the key. If your receipts for the job are already logged in the same thread, you're not hunting for numbers; the invoice is mostly assembled by the time you ask for it. The general pattern is the one in invoicing from your pocket: the faster the invoice goes out, the faster it gets paid — and invoices sent same-day, in person, get queried far less.
A job history for every vehicle, by thread
Mobile mechanics live on repeat custom, and repeat custom runs on history. When Mrs Patel calls about the Fiesta's brakes, the questions are always the same: what did I do last time, when, what parts went in, what did I flag as 'watch this'? A garage has a management system for that. You can have something just as useful with message threads — one running record per vehicle.
Every job on a vehicle adds to its record: date, registration, work done, parts fitted (with the receipt photos), mileage on the clock, and anything advisory — 'rear discs lipped, 6 months maybe'. Next time the phone rings, thirty seconds of scrolling tells you the whole service history. That does three things:
- Better quotes: you know the vehicle, so you price the follow-up work accurately instead of guessing.
- More work: those advisories are your pipeline — 'when I did your clutch in March I noticed the battery was weak' is the easiest sales call in the world.
- Protection: if a customer claims a fault relates to your work, a dated record of exactly what you did and fitted is your defence.
It's the same principle as keeping job records by text, applied per-vehicle — and it costs you one message at the end of each job.
The whole system, from the driver's seat
Receipt photo at the factors. Mileage line per leg. Invoice on the driveway. One closing message per vehicle. Four habits, each under a minute, all from the phone that's already mounted on your dash — and your January self-assessment becomes a sorting exercise instead of a forensic investigation.
GraftG turns one WhatsApp number into your back office: mileage, receipts, quotes, invoices and a job tracker, all by text, with no app to download. Your admin sorted. Just WhatsApp it. It's launching soon — join the waitlist at graftg.co.uk.