Allow adding / editing a customer's Tax ID (GST/VAT) on an already-issued invoice

Currently a customer's GSTIN (Business Tax ID) can only be captured at checkout. If it's missed or entered wrong, there's no way to add or correct it afterward — not via the dashboard, customer portal, or API (the invoice is read-only and there's no PATCH on a completed payment). This blocks Indian B2B customers from claiming input tax credit, since their GSTIN must appear on the tax invoice.

Requested feature: let the merchant (or customer via portal) add/edit the Business Tax ID on an issued invoice and regenerate the tax invoice with it ideally supported across dashboard, customer portal, and API.

Real example: merchant Arpit Bansal a ₹26,550 B2B payment (₹4,050 GST @18%) to a CA firm where the customer's GSTIN was missed at checkout, with no way to correct the invoice afterward.