Submitting my form to the database and then redirecting to payment site

Good afternoon, I managed to sort this out. I have amended my functions.php to the following.

function mcPaySubmit(){
global $wpdb;

include('paygate.php');
$last = wp_get_recent_posts(array('numberposts' => '1', 'post_type' => 'post', 'post_status' => 'pending', 'author' => $user_id));
$purchase_post_id = $last['0']['ID'];
$purchase_days = $_POST['dext'];
$purchase_date = $_POST['mcTransactionDate'];
$purchase_amount = $_POST['totalDue'];

if($wpdb->insert('ac_purchases', array(
    'purchase_post_id' => $purchase_post_id,
    'purchase_days' => $purchase_days,
    'purchase_date' => $purchase_date,
    'purchase_amount' => $purchase_amount,
    'purchase_status' => 'pending'
    ))===FALSE) {

    echo "ERROR";
    } else {

    $reference = $wpdb -> insert_id;
    $amount = str_replace('.','',$purchase_amount);
    $checksum_source = $PAYGATE_ID."|".$reference."|".$amount."|".$CURRENCY."|".$RETURN_URL."|".$purchase_date."|".$EMAIL."|".$encryption_key;
    $CHECKSUM = md5($checksum_source);
    $response = array('AMOUNT' => $amount, 'TRANSACTION_DATE' => $purchase_date, 'REFERENCE' => $reference, 'CHECKSUM' => $CHECKSUM, 'PAYGATE_ID' => $PAYGATE_ID, 'CURRENCY' => $CURRENCY, 'RETURN_URL' => $RETURN_URL, 'EMAIL' => $EMAIL);
    echo json_encode($response);
    }
    wp_die();
}
add_action('wp_ajax_mcPaySubmit', 'mcPaySubmit');
add_action('wp_ajax_nopriv_mcPaySubmit', 'mcPaySubmit');

my form to this:

<script>
function payCalc(){
    var dext = document.getElementById("dext").value;
    var sumDext = document.getElementById("sumDext").value = Math.round(24.99 * dext * 100)/100;
    var totalDue = document.getElementById("totalDue").value = Math.round((sumDext + 79.99) * 100)/100;
}
</script>

<form method="POST" id="mcPayment" name="mcPayment" action="https://www.paygate.co.za/paywebv2/process.trans">
<table>
    <tr>
        <td colspan="3"><b>Please note that the amounts are quoted in South African Rands (ZAR)</b></td>
    </tr>
    <tr>
        <td colspan="2"><label for="mcPublish"><b>Publish for 3 Days</b></label></td>
        <td><input type="text" id="mcPublish" name="mcPublish" readonly value="79.99" /></td>
    </tr>
    <tr>
        <td><label for="daysToExtend" /><b>Number of Days to Extend</b></label></td>
        <td><input type="number" id="dext" oninput="payCalc()" name="dext" min="0" value="1" /></td>
        <td><input type="text" id="sumDext" name="sumDext" readonly value="24.99" /></td>
    </tr>
    <tr>
        <td colspan="2"><label for="totalDue"><b>Total Due<b></label>
        <td><input type="text" id="totalDue" name="totalDue" readonly value="104.98" /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="mcTransactionDate" name="mcTransactionDate" value="<?=$TRANSACTION_DATE?>"></td>
        <td><input type="hidden" id="my-hidden-field" name="my-hidden-field" value="this is hidden" />
    </tr>
    <tr>
        <td><input type="hidden" name="action" value="mcPaySubmit" /></td>
        <td><input type="submit" id="mcPaySubmit" name="mcPaySubmit" value="Proceed to Pay" /></td>
        <td></td>
    </tr>
    <tr>
        <td><input type="hidden" id="PAYGATE_ID" name="PAYGATE_ID" value="<?=$PAYGATE_ID?>" /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="REFERENCE" name="REFERENCE" value="<?=$REFERENCE?>" /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="AMOUNT" name="AMOUNT" value="<?=$AMOUNT?>") /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="CURRENCY" name="CURRENCY" value="<?=$CURRENCY?>" /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="RETURN_URL" name="RETURN_URL" value="<?=$RETURN_URL?>" /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="TRANSACTION_DATE" name="TRANSACTION_DATE" value="<?=$TRANSACTION_DATE?>" /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="EMAIL" name="EMAIL" value="<?=$EMAIL?>" /></td>
    </tr>
    <tr>
        <td><input type="hidden" id="CHECKSUM" name="CHECKSUM" value="<?=$CHECKSUM?>" /></td>
    </tr>
</table>
</form>

and the js to this.

$ = jQuery.noConflict();

$(document).ready(function() {
$('#mcPayment').submit(submit_mcPayment);
});

function submit_mcPayment(){

$.ajax({
    url: myAjax.ajaxurl,
    type: "POST",
    data: $(this).serialize(),
    dataType: 'json',
    success: function mcPaymentSuccess(response) {
            $('#AMOUNT').val(response.AMOUNT);
            $('#TRANSACTION_DATE').val(response.TRANSACTION_DATE);
            $('#REFERENCE').val(response.REFERENCE);
            $('#CHECKSUM').val(response.CHECKSUM);
            $('#PAYGATE_ID').val(response.PAYGATE_ID);
            $('#CURRENCY').val(response.CURRENCY);
            $('#RETURN_URL').val(response.RETURN_URL);
            $('#EMAIL').val(response.EMAIL);
            document.forms['mcPayment'].submit();
        }
    });
    return false;
}