all form processing must occur on the forms action page.
you have the action of the form set to post to another page:
<form class="NewFromBlank_Default" id="financingform_NewFromBlank_Default" name="financingform_NewFromBlank_Default" method="post" action="inventory.php">
but the UE behavior is on the form page.
change the action to post to the same page as the form:
<form class="NewFromBlank_Default" id="financingform_NewFromBlank_Default" name="financingform_NewFromBlank_Default" method="post" action="boats_detail.php">
In UE< you can set the go to page to redirect after the mail is sent.