Adding a WhatsApp chat button that floats on every page

You can prefill the message too. Add ?text= to the end of the click-to-chat link with your message URL-encoded (spaces become %20), something like "Hi, I have a question about my order". Customers find it less awkward to start, and you can tell which chats came from the button. I'd hide it on the checkout page as well. One more distraction there is not what you want.

You don't need an app for this. A small snippet in theme.liquid just before the closing body tag does it: a link to WhatsApp's click-to-chat address with your number in full international format (country code, no plus, no leading zero), wrapped in a div with position: fixed, bottom and right around 20px, and a high z-index. Two things people miss: test it on mobile, because it can sit on top of the add to cart bar or a cookie banner, and remember that editing theme.liquid directly means the change won't carry over when you update or switch themes.