Enter your GSTIN in the billing address during checkout, or save it on the address in Dashboard → Addresses so it is filled in automatically next time. The invoice is a statutory document, so once it has been issued the GSTIN on it cannot simply be edited. If you missed it, raise a support request with your order number and GSTIN. We will issue a credit note and a corrected invoice, which is the correct treatment under GST. Which tax appears depends on where you are: a customer in Uttar Pradesh, where we are registered, is charged CGST plus SGST. Everywhere else in India is charged IGST at the same combined rate.