Disable unload function when submitting form (JQuery)

Question

I'm still a beginner in Jquery. What I am trying to do here is when user try to submit the form, the unload function will be disabled.

This is check out form.

<form name="checkoutform" method="POST" action="checkcard.php">

Card Holder: <input type="text" class="input" placeholder="Holder Name" name="cardholder" required>
Card Number: <input type="text" class="input" placeholder="Card Number" data-mask="0000 0000 0000 0000" name="cardnumber" required>
Card Expiry Data: <input type="text" class="input" placeholder="00 / 00" name="expirydate" required>
<input type="submit" name="checkoutbtn" value="CHECKOUT NOW">

</form>

This is the unload script.

<script>//delete reserve if browser is closed

    $(window).unload(function(){
        window.open("deletereserve(no_redirect).php?reserveid=<?php echo $reserveid ?>");
    })

</script>

This is my deletereserve php file

<?php
    include "connection.php";
    session_start();

    $reserveid = $_GET["reserveid"];

    $delete = "DELETE FROM reservation_seat where Reservation_ID = $reserveid";
    mysqli_query($connect, $delete);

    $delete2 = "DELETE FROM ticket_reservation where Reservation_ID = $reserveid";
    mysqli_query($connect, $delete2);

    echo "<script>window.close();</script>";
?>