How would I convey the information that multiple options can be selected, and just one can be selected as well?

So, this is what my scene looks like:

How would I show the users that they can book only one service, or more than one? I wouldn't want to make a clickable '+' instead of a book button that adds it to a list and at the end you have only one book button, because most of the times users would just book one service.

I thought of having a '+' like the last paragraph says but it only takes 1/3 of the 'book panel' (the black background behind the book text) when someone clicks it then the book services comes up, showing the users they can select various services.

Is the way I thought of alright? Or maybe the idea I'm trying not to recreate is not so critical for the user experience?

Thanks