Add sales tax to docs (#35059)

Release Notes:

- N/A

docs/src/ai/billing.md

@@ -25,3 +25,12 @@ From Stripe’s secure portal, you can download all current and historical invoi
 You can update your payment method, company name, address, and tax information through the billing portal.
 
 Please note that changes to billing information will **only** affect future invoices — **we cannot modify historical invoices**.
+
+## Sales Tax {#sales-tax}
+
+Zed partners with [Sphere](https://www.getsphere.com/) to calculate indirect tax rate for invoices, based on customer location and the product being sold. Tax is listed as a separate line item on invoices, based preferentially on your billing address, followed by the card issue country known to Stripe.
+
+If you have a VAT/GST ID, you can add it at [zed.dev/account](https://zed.dev/account) by clicking "Manage" on your subscription. Check the box that denotes you as a business.
+
+Please note that changes to VAT/GST IDs and address will **only** affect future invoices — **we cannot modify historical invoices**.
+Questions or issues can be directed to billing-support@zed.dev.