my shopping cart does not capture the products on the first attempt

[BlackKwazi]

if(isset($_POST["add_to_cart"])){

    if(isset($_SESSION["shopping_cart"])) {
        $item_array_id = array_column($_SESSION["shopping_cart"],"item_id");
        if(!in_array($_GET["ItemID"],$item_array_id)){

            $count = count($_SESSION["shopping_cart"]);
            $item_array = array(
                'item_id' => $_GET["ItemID"],
                'item_name' => $_POST["hidden_ProductName"],
                'item_price' => $_POST["hidden_ProductPrice"],
                'item_quantity' => $_POST["quantity"]
            );
            $_SESSION["shopping_cart"][$count] = $item_array;
        }
        else{
                echo '<script>alert("item already added, pick a different item")</script>';
                echo '<script> window.location ="productPage.php" </script>';
        }
    }
    else{
            $item_array = array(
                'item_id' => $_GET["ItemID"],
                'item_name' => $_POST["hidden_ProductName"],
                'item_price' => $_POST["hidden_ProdcutPrice"],
                'item_quantity' => $_POST["qunatity"]
            );
            $_SESSION["shopping_cart"][0] = $item_array;
    }

[BlackKwazi]

$values){ ?>

                    <tr>
                        <td><?php echo $values["item_name"]; ?></td>
                        <td><?php echo $values["item_quantity"]; ?></td>
                        <td>R <?php echo $values["item_price"]; ?></td>
                        <td>R <?php echo number_format($values["item_quantity"] * $values["item_price"], 2); ?></td>
                        <td><a href="productPage.php?action=delete&ItemID=<?php echo $values["item_id"]; ?>"><span class="text-danger"> Remove </span></a></td>
                    </tr>
                    <?php 
                            $total = $total + ($values["item_quantity"] * $values["item_price"]);     
                        }
                    ?>
                    <tr>
                        <td colspan="3" align="right">Total</td>
                        <td align="rigt">R<?php echo number_format($total,2); ?> </td>
                        <td></td>
                    </tr>
                   <?php
                        }
                    ?>