Can customers pay inside WhatsApp, and in which countries?

Yes, but only in four countries. WhatsApp's Help Center lists India, Brazil, Mexico and Indonesia as places where customers can pay registered businesses inside WhatsApp.

• India: after ordering, the customer taps Review order, then Pay now, and enters their UPI PIN.
• India, restaurant side: the API sends an order message. Razorpay, PayU, Billdesk and Zaakpay are supported directly, and any UPI Intent gateway works.
• Brazil: Pix codes, payment links, boleto and card payments. WhatsApp does not reconcile payments, so you match them in your payment provider.
• Mexico and Indonesia are on WhatsApp's list, but Meta's developer payment guides cover only India and Brazil. Ask your provider.
• Everywhere else, including the UAE, Saudi Arabia, the UK and the US, send a payment link or take cash.