When an invoice comes out wrong, the useful question is not what is wrong with it but which setting produced it. Almost nothing on a GST invoice is typed per bill — most of it is pulled from three places you filled in earlier.
The header — from your business settings
Your name, address, GSTIN and logo. If the header is wrong on one invoice it is wrong on all of them, because none of it is entered per bill. Fix it in settings, not on the invoice.
The customer block — from the customer record
Name, address, GSTIN, and place of supply. Change it on the customer and every future invoice follows; change it on one invoice and you have created a discrepancy between the bill and your own records. Always fix the record.
The line items — from your item master
Description, HSN or SAC, and the rate. The rate travels with the item, not the invoice, which is exactly why it should be set once per item and never chosen per bill. If one product is being taxed inconsistently across invoices, the fault is that someone is overriding it per bill.
The tax split — computed, not chosen
IGST, or CGST plus SGST. Nobody types this. It falls out of your state and the customer's state, both of which came from GSTINs. If the split looks wrong, one of those two GSTINs is wrong — that is nearly always the whole diagnosis.
The declaration block — from your GST profile
Things like an LUT number for export or SEZ supply, or a reverse-charge note, appear automatically when the relevant setting is switched on. They are not optional text you remember to add; they are a consequence of how your profile is configured.
The payment block — from your UPI ID
Present if you saved one, absent if you did not. It is the only part of the invoice whose job is to get you paid faster rather than to satisfy a record-keeping requirement.
How to debug an invoice in one pass
- Header wrong → business settings.
- Customer details wrong → the customer record.
- Rate or HSN wrong → the item.
- IGST vs CGST+SGST wrong → one of the two GSTINs.
- Declaration missing → the GST profile toggle for it.
- No way to pay → no UPI ID saved.
A note on what this article does not say
No rate, threshold, due date or penalty appears anywhere above, deliberately. Those come from the GST portal or your accountant, with the source recorded, and any software article that recites them from memory is doing you a disservice — including when it happens to be right this year.