<span id="price">900</span>
<form id="contruct-from">
   <input name="namecontr">
   <input name="telcontr">
   <input name="emailcontr">
   <button form="contruct-from" type="submit">ОТПРАВИТЬ</button>
</form>

$('.btn-contruct').click(function(){
    if($("#contruct-from")[0].checkValidity()) {
        $.post('Post_Constructor.php', '&item_price=' + $('#price').text() + $('#contruct-from').serializeArray(), function(data) {

        });
            console.log('ok');
            return false;

        }
    });

<?php
session_start();

new post_contact($_POST);

class post_contact
{

public function __construct($post_data)
{
    $this->send_contact_form($post_data);
}

function send_contact_form($post_data)
{
$headers = "MIME-Version: 1.0" . PHP_EOL .
"Content-Type: text/html; charset=utf-8" . PHP_EOL .
'From: test@email.com <test@email.com>' . PHP_EOL;

$message = '<p>'.$_POST['namecontr'].'</p>';
$message .= '<p>'.$_POST['telcontr'].'</p>';
$message .= '<p>'.$_POST['emailcontr'].'</p>';
$message .= '<p>'.$_POST['item_price'].'</p>';

mail('test@email.com', 'Помогите пожалуйста', $message, $headers );
}
}

$('#contruct-from').serialize()

$('#contruct-from').serializeArray();

$.post('Post_Constructor.php',  'item_price=' + $('#price').text() +'&'+ $('#contruct-from').serialize(), function(data) {});