Invoices and GST
Every successful payment generates an invoice automatically. You'll find them all under More → Billing, each one downloadable as a PDF.
Add your billing details first
Invoices are generated from whatever details are on your account at the time of the charge, so it's worth filling these in before you upgrade rather than after. They live under More → Account: your name, address, country and state.
Country and state are pre-filled from your approximate location as a convenience, but they're only a starting guess. Check them. They determine how tax is applied, and an invoice can't be silently rewritten once it has been issued.
If you have a GSTIN
Add it in the same place and tick the GST invoice option. Your GSTIN determines the place of supply, so the state is taken from the number itself rather than from anything you type separately, which is exactly how it's meant to work.
For Indian customers, invoices show the correct split automatically: CGST and SGST when the place of supply matches ours, IGST when it doesn't. You don't have to choose, and you can't choose wrong.
If you're outside India
Prices are shown and charged in US dollars, and no Indian GST is added. The invoice is issued as an export of services.