We register you with the provider YALST at the free rate, set up your account and build a chat window in your shop. When you are online, the customer can see this, click on the window and chat directly with you.
When you are offline, the customer is redirected to a contact form and can write you a message.
There are countless templates for the online/offline view, a few of which you can see here as photos. Of course you don't have to decide right away, you can simply log in later and change the template with a mouse click or upload your own graphic, no technical knowledge is required for this.
The basic version of Yalst Chat is free to use!