GST has been part of Indian business life since July 2017. But for most business owners — especially those running pharmacies, hotels, restaurants, retail shops, or service businesses — the day-to-day compliance requirements remain a source of anxiety: Am I using the right tax type? Are my invoice numbers sequential? Do I need to e-invoice? What happens if I get it wrong?
This guide answers every practical question about GST billing for Indian businesses in 2025 — and explains how modern billing software handles the complexity so you can focus on running your business.
The Basics: What GST Is and How It Works
GST (Goods and Services Tax) is a destination-based tax on the supply of goods and services in India. It replaced a complex web of state and central taxes — VAT, service tax, excise duty, entry tax — with a unified framework.
GST is collected at multiple rates: 0%, 5%, 12%, 18%, and 28%, depending on the product or service category. Some items are exempt. Some are in a separate cess category.
GST flows through the supply chain via ITC (Input Tax Credit): A business pays GST when it buys inputs. It collects GST when it sells outputs. The GST payable to the government is the output tax minus the input tax already paid. This means every invoice in the chain needs to be correct — because ITC claims depend on valid tax invoices.
CGST, SGST, and IGST: Knowing Which Applies
The most common source of GST billing confusion is whether to apply CGST + SGST or IGST.
Intra-state supply (seller and buyer both in the same state):
- Apply CGST + SGST in equal halves
- For 18% GST: 9% CGST + 9% SGST
- For 5% GST: 2.5% CGST + 2.5% SGST
Inter-state supply (seller and buyer in different states):
- Apply IGST only (not CGST + SGST)
- For 18% GST: 18% IGST
- For 5% GST: 5% IGST
How to determine the buyer's state: For B2B transactions, the buyer's GSTIN contains their state code as the first two digits. A GSTIN starting with "06" is Haryana. "27" is Maharashtra. Your billing software should detect this automatically and apply the correct tax type.
Applying the wrong type — CGST+SGST on an inter-state sale, or IGST on an intra-state sale — creates reconciliation issues in GSTR-2A that your accountant will need to resolve. With proper software, this never happens.
What a Valid GST Invoice Must Contain
A GST invoice that is missing required elements may not be accepted for ITC by the buyer. For B2B transactions, this is a serious problem — your customer won't be able to claim back the tax they paid.
A valid GST tax invoice must include:
- Supplier details — registered name, address, and GSTIN
- Invoice number — unique, sequential within a financial year, no gaps
- Invoice date
- Buyer details — name, address, and GSTIN (for registered buyers)
- Place of supply — the state where delivery occurs, determines CGST/SGST vs. IGST
- HSN/SAC code — for each item or service
- Description, quantity, and unit of measurement
- Unit price and total value before tax
- Discount (if any, shown separately)
- Tax rate and tax amount — CGST and SGST separately, or IGST
- Total invoice value
- Signature — physical or digital
For retail B2C transactions (consumer purchases), the GSTIN of the buyer is not required. A simplified bill of supply is acceptable for transactions below ₹200.
E-Invoicing: Who It Applies to in 2025
E-invoicing is not a separate document type — it's a process by which GST invoices are authenticated by the Invoice Registration Portal (IRP) before they're shared with the buyer.
Current mandate threshold (2025): Businesses with annual aggregate turnover above ₹5 crore must generate e-invoices for all B2B transactions.
How e-invoicing works:
- The seller generates an invoice in their billing software
- The invoice data is uploaded to the IRP (Invoice Registration Portal)
- IRP validates the data and returns an IRN (Invoice Reference Number) and a digitally signed QR code
- The seller embeds the QR code on the invoice and shares it with the buyer
- The buyer can verify the invoice using the QR code
If an eligible business issues an invoice without getting it registered on the IRP, the invoice is considered invalid for ITC purposes. The buyer cannot claim input tax credit on it.
What this means for your billing software: GoClixy handles IRP integration for eligible businesses automatically. You generate the invoice in GoClixy, the system registers it with IRP, and the returned IRN and QR code are embedded in the printed invoice — without any manual API interaction on your part.
HSN Codes: Getting Product Classification Right
Every product sold in India must be classified under an HSN (Harmonised System of Nomenclature) code. This code determines:
- The applicable GST rate
- How the sale is reported in GSTR-1
- Whether the transaction is eligible for certain exemptions or concessions
Common HSN code mandates:
- Businesses with turnover above ₹5 crore must show 8-digit HSN codes on invoices
- Businesses with turnover ₹1.5 crore to ₹5 crore need 4-digit HSN codes
- Businesses below ₹1.5 crore need 2-digit HSN codes (recommended to use 4-digit for accuracy)
Getting the HSN code wrong doesn't just create a compliance issue — it can mean charging the wrong GST rate, which creates either tax liability (if you under-charge) or pricing problems (if you over-charge and then have to refund the difference).
GoClixy maintains HSN and SAC code libraries for the product categories relevant to each industry — pharmacy, restaurant, hotel, retail, garment, and more — so the correct code is assigned automatically when you add a product.
GSTR-1 and GSTR-3B: Your Monthly Filing Obligations
Most GST-registered businesses file:
GSTR-1 (outward supply return): Details of all sales invoices issued in the month. Due by the 11th of the following month for monthly filers, or quarterly for businesses under QRMP scheme.
GSTR-3B (summary return and tax payment): Summary of output tax, input tax credit claimed, and net tax payable. Due by the 20th of the following month.
The data for both returns comes directly from your billing and purchase records. If these are maintained in GoClixy, the GSTR-1 and GSTR-3B data is exported automatically — classified correctly by tax type, rate, and transaction type (B2B, B2C, export, etc.).
If they're maintained in Excel, someone needs to compile and classify all transactions manually — which takes hours and introduces errors that create mismatches between your filing and the auto-populated data in the GST portal.
Common GST Compliance Mistakes and How to Avoid Them
Wrong tax type on inter-state sales: Applying CGST+SGST when IGST should apply, or vice versa. Avoided by software that reads the buyer's GSTIN and applies the correct type automatically.
Invoice number gaps: GSTN flags discontinuous invoice number sequences. Avoided by software that maintains a tamper-proof sequential series.
Wrong GSTIN on buyer invoices: If the buyer's GSTIN is incorrect, their ITC claim will fail. GoClixy validates GSTINs against the GSTN API in real time at the time of entry.
Late ITC claims: Input tax credit must be claimed by the filing of the return for November (for the previous financial year). GoClixy's purchase register ensures all purchases are recorded with correct vendor GSTINs for timely ITC reconciliation.
Missing credit notes: When a sale is reversed, a credit note must be issued (not a "cancelled invoice"). GoClixy generates linked credit notes that adjust the original invoice data in GSTR-1 automatically.
Credit Notes and Debit Notes
When an invoice needs to be revised after issue, the correct mechanism is a credit note (to reduce the amount) or a debit note (to increase it). These must reference the original invoice.
Circumstances requiring credit notes:
- Goods returned by the buyer
- Price reduction agreed after invoice
- Discount applied retrospectively
- Invoice value correction (overcharge)
GoClixy generates GST-compliant credit and debit notes linked to the original invoice, with the correct date, reference, and tax adjustment — ensuring your GSTR-1 reflects the net position accurately.
→ See How GoClixy Handles GST Automatically →
Frequently Asked Questions
What is the difference between CGST, SGST, and IGST? CGST + SGST apply to intra-state transactions (same state, split equally). IGST applies to inter-state transactions (different states, full rate). Your billing software should detect and apply the correct type automatically from the buyer's GSTIN.
What must a valid GST invoice contain? Supplier GSTIN, sequential invoice number, date, buyer GSTIN (B2B), HSN/SAC codes, itemised amounts, CGST/SGST or IGST amounts, and total value. Missing elements can invalidate the invoice for buyer's ITC claim.
Who needs to generate e-invoices in India in 2025? Businesses with annual aggregate turnover above ₹5 crore must register all B2B invoices on the IRP before sharing with buyers. GoClixy handles IRP integration automatically for eligible businesses.
What is an HSN code? The product classification code that determines GST rate and GSTR-1 reporting. GoClixy maintains HSN libraries per industry so the correct code is assigned automatically.
What happens if a GST invoice has an error? Issue a credit note (to reduce) or debit note (to increase). GoClixy generates these linked to the original invoice with automatic GSTR-1 adjustment.
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Also read: 5 Signs Your Business Has Outgrown Excel · Medical Store Schedule H and GST Compliance