this is how i would approach it:
in your process script, lets say process.php, i would loop through your $_POST and check it against an array that holds the names of the possible add-ons, to see if it is set. for each of the possible add-ons that are selected, i would put those into an array. then you have all of the options the user selected in an array
heres some pseudo-code:
possible_add_ons = array(foo, bar, widget, etc)
selected_add_ons = array()
foreach(possible_add_on as key){
if(array_key_exists(key, $_POST)){
selected_add_ons .= key
}
}
something to that extent. hope this helps or at least points to the right direciton
