Add unique field in your form.
Eg: <input type="hidden" name="wdm_action" value="1">
Put your 'customer-details.php'
file code in below function:-
add_action('template_redirect','wdm_customer_details'){
//check if that field present in the form, if yes then perform the action
if(isset($_POST['wdm_action'] && $_POST['wdm_action'] != ''){
//customer-details.php file code
}
}