If you have more than one extra options and you wish to be able to check only one you need to make the below changes:

In  help_functions.php in function wpestate_show_extra_options_booking, replace checkbox with radio as per the below screenshot

After you will need to adjust the radio buttons with custom css

.cost_explanation input[type=”radio”] {
height: 20px;
width: 20px;
float: left;
margin-right: 5px;
margin-bottom: 0px!important;

The CSS needs to be added in Theme Options-Design-Custom Colors-Custom CSS or Child Theme.