AJAX在表单重定向到Paypal之前验证并将数据发送到DB

Question

I would like to create Validation using AJAX and update DB before form is sent to Paypal.

<form method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr" class="paypal-button" target="_top">
    <div class="hide" id="errorBox"></div>
    <input type="hidden" name="env" value="www.sandbox"><input type="hidden" name="notify_url" value="www.immortalinsurance.com">
    <input type="hidden" name="tax" value="0">
    <input type="hidden" name="shipping" value="0">
    <input type="hidden" name="currency_code" value="AUD"><input type="hidden" name="amount" value="$15.00">
    <input type="hidden" name="quantity" value="1">
    <input type="hidden" name="item_name" value="Cover Yourself &amp; a Friend"><input type="hidden" name="custom" value="">
    <p class="paypal-group">
        <label class="paypal-label" for="policy_name1">policy_name1
            <input type="text" class="paypal-input" name="policy_name1">
        </label></p>
    <p class="paypal-group">
        <label class="paypal-label" for="policy_name2">policy_name2
            <input type="text" class="paypal-input" name="policy_name2">
        </label></p>
    <input type="hidden" name="button" value="buynow"><input type="hidden" name="cmd" value="_xclick">
    <input type="hidden" name="business" value="[e-mail]"><input type="hidden" name="bn" value="JavaScriptButton_buynow">
    <button type="submit" class="paypal-button large">Buy Now</button>    
</form>

1, Make sure fields are NOT empty, else error.

2, Verify that the price matches $row['price']; else error.

if above statements are true then update DB and allow form to continue to paypal.

All i have at the moment is :

$(document).on('submit', '.additem-form', function(event){

    var self = this;

    event.preventDefault();

    $.ajax({

                type: "POST",

                url: "controller/initialstoring.php",

                data: {

                    itemid: $('input[name=itemid]').val(),
                    amount: $('input[name=amount]').val()

                },

                success: function(data) {

                    $('input[name=custom]').val($lastid);

                    Continue with Usual Form Submit...
                }
    });
});     

Here is my initialstoring.php:

<?php

include("../application/config/db_config.php");

$itemid = $_POST['itemid'];
$date = date('d-m-Y h:i:s');
$itemprice = $_POST['itemprice'];

$sql = "SELECT * FROM policies WHERE id = $itemid";
$stmt = $mysqli->query($sql);
$row = mysqli_fetch_assoc($stmt);


if ($itemprice == $row['price']) {
    $insert_row = $mysqli->query("INSERT INTO purchases (name_on_policy, other_on_policy, dateofpurchase, status) VALUES ('$certname', '$othercertname', '$date', '$status')");
    $lastid = $mysqli->insert_id;
} else {
    echo "error";
}

?>

Is this the correct method in doing this? Also would there be any reason to check the price is = $row[price] before submit?

How do i after ajax is Success and just before actual form submit, add to the custom variable?

Thanks