The number to watch
Returns as a share of sales. Auto parts are returned when the wrong item was identified, and every return costs handling and often goodwill. The figure tells you whether the problem is at the counter or in the catalogue. Recording the part number on every bill makes both the return and its cause traceable — usually a small set of items where the numbering is confusing, which is fixable once you know which they are.
Part number is the item
An auto parts shop lives or dies on identifying the right part, and the part number is what does it. Keeping it as the item code — searchable and scannable — means a counter enquiry is answered in seconds rather than by opening a catalogue. Vehicle model and make belong on the item too, because that is how the customer will ask, and it is what prevents the wrong part being sold and returned.
Genuine, OEM and aftermarket
The same part exists at three quality levels and three price points, and a shop that enters them as one item cannot track either stock or margin. Each as its own item, clearly named, also protects the conversation at the counter — the customer chose the cheaper one and the bill says so, which matters when it fails in eight months and they come back.
Mechanic and garage credit
Your regular buyer is a mechanic drawing parts for jobs he will bill at the end of the week or month. Small amounts, high frequency, and a running account that nobody totals. The ledger updating from the billing means it is recorded without effort, and a weekly or monthly statement settles it before it becomes a number both sides argue about.