Create a select list where the values are the email addresses to send to:
<select name="sendTo">
<option value="sales@domain.com">Sales</option>
<option value="service@domain.com">Service</option>
<option value="support@domain.com">support</option>
</select>
then on the contactus.php page, change line 37:
$RecipientEmail = "".($WAGLOBAL_Contact_Email_To) ."";include("WA_Universal_Email/WAUE_contact_1.php");
to:
$RecipientEmail = "".(isset($_POST['sendTo'])?$_POST['sendTo']:"") ."";include("WA_Universal_Email/WAUE_contact_1.php");