Two things on one bill
A workshop invoice is almost always two supplies at once: labour, which is a service, and parts, which are goods. They carry different GST rates — repair and maintenance services sit at one rate, most parts at a higher one — and the tax authorities have been clear that where a workshop shows the value of parts and the value of labour separately, each is taxed at its own rate. So the first working rule is the simplest: parts and labour go on separate lines, always. A bill that says \u201cservice — twelve thousand\u201d and nothing else is the bill that gets asked about.
GST invoice or Bill of Supply
If you are registered under the regular scheme, every taxable sale needs a GST invoice: your GSTIN, the customer's if they have one, HSN or SAC codes per line, the taxable value and the tax split into CGST and SGST, or IGST for another state. Where GST does not apply to what you are billing — an exempt supply, or a business under the composition scheme, which cannot charge tax on its bills — you issue a Bill of Supply instead. The document type is not a style choice; the wrong one on the wrong sale is a compliance problem months later. A system that raises the right document from the job card removes the choice from the person at the counter.
Input credit is where the money is
Every part you buy from a registered supplier carries GST you have already paid. Under the regular scheme that tax is input credit, set off against the tax you collect on your own bills — provided you hold a proper purchase invoice and the supplier has reported it. For a workshop whose parts bill is a large share of turnover, this is the single biggest reason to register before the threshold forces you to. It is also why purchase invoices belong in the system against the vendor, not in a carton: the credit you cannot document is credit you do not get.
The month, and the year
- Every month or quarterOutward supplies go into GSTR-1 — invoice by invoice for registered customers, summarised for retail — and the tax summary and payment into GSTR-3B. Smaller businesses can file GSTR-1 quarterly under the QRMP scheme while paying monthly.
- What the CA needs from youA clean invoice-level export split B2B and B2C with the tax breakup, the purchase invoices for input credit, and the expense ledger. If your system produces these as files the CA can import, month-end is an email; if it produces a bag of counterfoils, month-end is a week.
- Year-endThe annual return and a reconciliation of what you filed against your books. The businesses that sail through it are the ones whose invoices, payments and purchases lived in one place all year.
Three habits that prevent most notices
One: never issue a manual bill outside the system for a \u201csmall\u201d job — the series break is what an assessing officer notices first. Two: record part-payments and balances against the invoice, so collected tax and billed tax reconcile. Three: send your CA exports, not screenshots. Rates, thresholds and scheme limits change, so confirm the current numbers with your CA; the three habits do not change.