WhatsApp supports a public click-to-chat link: put a phone number in full international format into a wa.me URL and the browser opens a conversation with that contact. Turn that link into a QR code and a customer can scan a sticker on a shop counter, a product label, a flyer or an invoice and land straight in a chat with your business.
The number must include the country code with no plus sign, spaces, brackets or leading zero - for example 447700900123 rather than +44 (0)7700 900123. That single formatting rule is the reason most click-to-chat codes fail. You can also append a pre-filled message with the text parameter, which is useful for 'I'd like to reorder', 'Request a quote' or 'Report a problem with order...' flows.
Everything is generated locally in your browser. The code is static, costs nothing to keep alive, and never expires. Add a short caption next to the printed code - 'Scan to message us' - because a bare QR symbol gives people no idea what will happen when they scan it.