GST Rule-46 Tax Calculation Engine

Indian GST operates under a dual-tax structure. When an order is created, SuperGST compares the Merchant Origin State Code with the Customer Shipping Destination State Code:

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  Intra-State Supply (Origin State = Destination State):

• The applicable GST rate is split equally into CGST (Central Tax, 50%) and SGST (State Tax, 50%).
• Example: On an 18% item, CGST = 9% and SGST = 9%.
• Reconciling Split: To prevent fractional rounding drift, SuperGST calculates CGST as round2(totalTax / 2) and SGST as round2(totalTax - cgst), guaranteeing that cgst + sgst === totalItemGst.

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  Inter-State Supply (Origin State ≠ Destination State):

• The entire tax rate is levied as IGST (Integrated Tax, 100%).
• Example: On an 18% item, IGST = 18%.

Most Shopify stores in India display retail consumer prices inclusive of all taxes (MRP). When calculating net taxable turnover for legal invoicing and GSTR reporting, the tax is reverse-calculated:

• Taxable Value Formula:

Taxable Value = round2(Gross Item Price ÷ (1 + (GST Rate ÷ 100)))

• Total Item Tax Formula:

Total Item Tax = round2(Gross Item Price - Taxable Value)

Example: A consumer buys an item priced at ₹1,180 (inclusive of 18% GST).

• Taxable Value: ₹1,180 ÷ 1.18 = ₹1,000.00
• Tax Amount: ₹1,180 - ₹1,000 = ₹180.00 (₹90.00 CGST + ₹90.00 SGST for local delivery).

Tax-Excluded (Wholesale / B2B Pricing):

If your store prices items exclusive of tax:

• Taxable Value: round2(Gross Item Price)
• Total Item Tax: round2(Taxable Value × (GST Rate ÷ 100))

In Indian e-commerce, shipping charges may either be exempt or subject to statutory GST depending on the merchant's business registration and logistics setup.

SuperGST gives merchants explicit control in Settings > Tax Rates & Shipping:

• Charge GST on Shipping Fees: When toggled off, delivery charges are treated as 0% GST (tax-exempt shipping), and the shipping charge is added directly to net taxable turnover without additional tax.
• Configured Shipping Tax Rate: When toggled on, shipping fees are taxed at the merchant's selected rate (0%, 5%, 12%, 18%, or 28%; default 18%).
• State Split: If shipping tax is enabled, intra-state orders split shipping tax equally between CGST and SGST, while inter-state shipments apply IGST.
• Inclusive vs. Exclusive: For stores with tax-inclusive pricing, the shipping taxable amount and tax are reverse-calculated using the selected shipping GST rate.

• Section 15(3) CGST Act Compliance: Per-line Shopify discounts (discount_allocations) and order coupons are deducted from gross line items before taxable values are determined:

Line Net Price = max(0, round2((Quantity × Base Price) - Discount Amount))

This ensures merchants only pay tax on actual revenue collected, never on discounted amounts.

• Grand Total Rounding: Invoices round grand totals to the nearest integer (Math.round), recording the round-off adjustment as a separate line item.
• Indian Currency Words: Converts totals to legal text using Lakhs ($10^5$) and Crores ($10^7$) formatting: "Rupees One Lakh Twenty-Five Thousand Four Hundred and Fifty and Fifty Paise Only".