Does a customer messaging me first mean I can send them promotions on WhatsApp?

No. A customer who messages you first lets you reply to them. WhatsApp's business policy still asks for opt-in permission before you send promotions.

• The policy says you may contact people only if they gave you their number or username and you have their opt-in permission.
• Opt-in must clearly say the person agrees to hear from your business, and it must name your business.
• Opt-in can be collected by SMS, on a website, by phone, or in person or on paper.
• Respect every request to stop, block or opt out, straight away.