It would be useful that there is a permanent framework next to the online booking with info.

Examples:

* How the customer should book, in steps. Also consider “choose an extra service after the menu” (you could easily add it here as an end step). Since it appears next to it, the customer can simply follow the steps while booking.

* House Rules. Conditions in case of not showing up by appointment, etc...

This way, you can also provide more information about the online booking itself than the first pop-up window.

This frame must be optional and easy to modify in terms of text yourself.

(not as big as on the attached photo, of course, is just an example... Doesn't have to overshadow online booking)