The post variables ($POST['somevalue']) are passed from whatever you are calling your first HTML page with the checkboxes and all the text boxes are $POST['thatfieldsName'] holds the value of that text field the checkboxes use $POST['DiveEquipment'][Air_Sales] in this case an associative array now these $POST variable are avaliable on the mailer.php page which is why is suggested reassigning shorter name for ease of entry of variables so when I stated
$airsales = $_POST['DiveEquipment'][Air_Sales];
$divesales = $_POST['DiveEquipment'][Dive_Equipment_Sales];
now $airsales hold the value of the box which could even be blank. So take the short variable that I posted at the top of the page and within your message for email or concatenate them as so;
$message .="Air Sales: $airsales\n Dive Rental $diverental....
the dot just before the equal = sign will append or concatenate all that data to the mail message. Make sure there are no spaces in the variables like $POST{'DiveEquipment'][Air Sales] (this will not work), and they must not have spaces in the form like name="DiveEquipment[Air Sales]" (neither will this it must either have the spaces removed or add an underscore in both files. Does that help any? Right now take the form and comment out the mail part and echo all the checkboxes by running the form and check all the boxes and in the mailer form temporarialy add echos for the seven variables that I have reassigned or you could just echo the $POST['DiveEquipment'][Air_Sales];