make sure to name the select list ending in "[]", for example:
mySelect[]
this will create an array of the selected items.
then on the confirm page, create a hidden form element of the same name in the collection of hidden form elements using the implode function to convert the array to a comma separated list:
<input type="hidden" name="mySelect" value="<?php echo isset($_POST['mySelect'])?implode(", ",$_POST['mySelect']):""; ?>" />
now edit the store order summary behavior, bind the column that will store the selections to the hidden form element.