Scanning with a gun or with the phone
A USB barcode scanner plugged into the computer works directly in the billing screen: scan, and the line appears with quantity one. Scan the same item again and the quantity goes to two. On the Android app the phone camera does the same job with no extra hardware, which is what most small shops start with. Either way the rate, unit, HSN code and GST come from the item record, so the bill is complete as it is built rather than corrected afterwards.
Items that came without a barcode
A large part of the hardware trade arrives loose — nuts and bolts, fittings, wire cut to length, locally made goods with nothing printed on them. For those you generate your own barcode against the item and print a label, and from then on it scans like anything else. This is also how shops handle the same item from two suppliers at two rates: each becomes its own item with its own code, so the stock and the margin stay honest.
Stock that is correct because billing is correct
Every scan reduces stock, so the closing figure is a by-product of billing rather than a separate counting exercise. Set a minimum level on any item and you are warned before it runs out, which is the difference between ordering in time and telling a customer at the counter that you do not have it. For a shop with thousands of SKUs, this is the only way stock ever stays accurate — nobody is going to count ten thousand items by hand.
Where barcode billing is worth it and where it is not
Barcodes earn their place in shops with many small, similar items sold in ones and twos — hardware, electrical, fasteners, sanitary fittings, auto parts, stationery. They are worth much less to a cement and steel dealer, where the same eight items make up most of the turnover and typing the name is quicker than reaching for a scanner. Be honest about which you are before spending on hardware. Plenty of shops use barcodes for the small-items half of the counter and name search for the bulk lines, in the same bill, which is usually the right answer for a general hardware shop.