Load WordPress post content into DIV using AJAX

I’m really in difficulties,

I have a button:
<a href="javascript: addtocart('<?php echo get_the_ID();?>','1')" class="button add_to_cart_button product_type_simple add-to-cart">ADD TO CART</a>

And my js:

function addtocart(productid,quantity){
    jQuery.ajax({
        type:"GET",
        url:"wp-content/themes/Teafull/addcart.php",
        data:"productid="+productid+"&quantity="+quantity,
        success:function(html)
        {
			alert('Update Success');
			var sum = html;
			$("#total-cart span").text(html);
			$("#right-cart").empty();
			$("#right-cart").load("wp-content/themes/Teafull/cart-update.php");
        }
    });
}

My file cart-update.php:

<?php
/*
Template Name: Update Cart
*/
session_start();?>
<h3><span>Gi? hàng</span></h3>
	<?php
		if($_SESSION['sizeof'] == 0){
			echo '<div class="empty-cart"><p style="color: #49aba8;">No products in your cart<p><div class="clear"></div>';
					}
		else{
				$ok = 1;
					if (isset($_SESSION['cart']))
					{
						foreach ($_SESSION['cart'] as $k => $v)
						{
							if (isset($k))
							{
								$ok = 2;
								break;
							}
						}
					}
					if($ok == 2){
							$total = 0;
							foreach ($_SESSION['cart'] as $key => $value)
							{
								$item[] = $key;
							}
							$str = implode(",", $item);
							$total =0;
							$my_query = query_posts(array('post__in' => $item,'post_type'=> 'product'));
							global $post;
							foreach ($my_query as $post) {
							   $posts_by_id[$post->ID] = $post;
							}
							foreach ($item as $id) {
								if (!$post = $posts_by_id[$id]) continue;
							   setup_postdata($post);

							   $price = mtbxr_val('price');
							   $quantity = $_SESSION['cart'][$id];	

								echo "<ul class='cart_list product_list_widget'><li>";
									echo "<a href='"; echo the_permalink(); echo "'>";
									echo the_post_thumbnail();
									echo "<p>";echo the_title(); echo"</p></a>";
									echo "<p class='clear' id='price-sidebar'><b>Price:</b>?"; echo $price;
								echo "</p></li></ul>";
							}

				}
		?>
		<div class="clear"></div>
		<p class="buttons">
			<a href="https://localhost/tem5/?page_id=159" class="button">My cart →</a>
		</p>
		<div class="clear"></div>
		<?php }?>

I want when I click on the button “ADD TO CARD”, div #right-cart load file cart-update.php.
When I click on the button “ADD TO CARD”, the following error happen:

Fatal error: Call to undefined function query_posts() in C:\xampp\htdocs\tem5\wp-content\themes\Teafull\cart-update.php on line 32

But, If I refresh page, Fatal error doesn’t happen.

Could anyone please help me how to solve this problem?