When WooCommerce is active, an Order via WhatsApp button is added before or after the Add to cart button. The message is updated live while the shopper changes the quantity or selects a variation.
The {total_price} line is only added when the quantity is more than one, and lines with an empty value (for example a product without a price) are removed automatically.
Block themes: if your single product template is built with blocks and the button does not appear, add a Shortcode block with [chat_order_button] to the template.