1. The verbal-approval trap
You discovered extra work mid-job - a rotten joist, hidden damage, something the original quote couldn't have anticipated. You emailed a revised quote. The customer replied "yeah, go ahead" on WhatsApp. Job's done. Now they're saying they only agreed to the original scope and refusing to pay the £3,000 for the extras.
With WorkPaid
The customer accepts a formal amendment link before the extra work starts. Timestamped. In writing. The agreement is on record - no debate about what "go ahead" meant.