The unit of work in paint retail is a site, not a bill
A paint dealership that measures itself in invoices is measuring the wrong thing. A single three-bedroom flat produces six or seven separate purchases over five weeks: putty, primer, undercoat, topcoat, a shade correction, and whatever was underestimated. Each one looks like a small transaction. Together they are one job worth a great deal more, and it can be lost at any point in the middle without anyone noticing.
Once the site is the record, the questions become answerable. How many litres has this site taken against its estimate? Which phase is it in? Has it stopped drawing? Those questions are invisible in a billing system.