SOLVED:
the email was not sending because of a problem: the select field has the form tag pointing to another form. Changing it to the actual form solved the problem.
Just to make things clear I've set the email SB trigger to my select field. Then I modified it this way:
<?php
if ((((isset($_POST["myselectfield"]))?$_POST["myselectfield"]:"") == "Express")) { //WA Universal Email
$Email = new WA_Email("myemail");...[/snip]