Meta bills your account directly for the messages your business starts: reminders, receipts, offers. To send those, a payment method must be on the account. Your first 1,000 replies each month to customers who message you first are free. After that, Meta charges for replies per message too.
What Meta accepts
A Visa or Mastercard credit or debit card. RuPay cards, UPI, net banking and Amex are not accepted for WhatsApp billing in India. If you only have a RuPay card, ask your bank for a Visa or Mastercard debit card, which most banks issue in a few days, or use a family member's card; Meta does not require the cardholder name to match the business.
Where to add it
In Meta Business Settings, Billing and payments, Messaging accounts, your account, Add payment method. Country India, currency INR. We do this with you on the setup call. Set a monthly spending limit there too, and raise it once you have seen a real bill.
How much it comes to
Meta charges per message for reminders and offers, and for replies after your first 1,000 in a month, plus 18 percent GST. Meta publishes its rates on its own rate card. The message-cost calculator on this site gives a rough estimate of yours.