in the add to cart button behavior, set the option column to be updatable from a select list.
after the form is created, edit the select list properties to enable to select multiple options. double check the name is edited to contain a [] at the end:
for example:
name="selectName[]"
then add the following code at line 1 of the page to convert the array of selections that is created to a comma seperated list:
if($_SERVER["REQUEST_METHOD"] == "POST") {
foreach($_POST as $key => $val) {
if(is_array($_POST[$key])) $_POST[$key] = implode(", ", $_POST[$key]);
}
}
?>