Serial number and warranty period
A battery claim is settled on the serial number and the date of sale, and the customer arrives with neither. Recording both on the invoice means you find it by party or item in seconds and process the claim with the manufacturer properly. This is the single most valuable record in the trade, because an unhonoured claim comes out of your margin.
Old battery exchange
The scrap value of the old battery is deducted from the sale, and doing that on a slip is how the final amount becomes disputable. Recording the exchange on the same invoice — what came in, at what value — makes the total explicable to the customer and gives you a record of scrap received, which is itself a stock item with a resale value worth tracking.
Inverters, installation and service
An inverter sale is a battery, a unit, wiring and an installation visit, each with its own margin. Billing them on separate lines keeps the hardware price comparable and the installation visible. Service calls afterwards — a top-up, a check, a replacement under warranty — are worth recording against the customer, because that history is what a repeat sale is built on.
The number to watch
Warranty claims honoured against claims made. Every claim you cannot trace to an invoice is a replacement you fund yourself, and most shops have no idea what that costs them in a year. Once serial and date are on every bill, that number goes to nearly all of them — which for a dealer selling volume is a serious saving that never appears on any report until you look.