Why Is jQuery Not Submitting This Form? (Or What Have I Done Wrong? :-p)

问题

HTML:

<form action="https://live.sagepay.com/gateway/service/vspform-register.vsp" method="POST" id="sagepay_form"> 
    <input type="hidden" name="navigate" value="" />
    <input type="hidden" name="VPSProtocol" value="2.23">
    <input type="hidden" name="TxType" value="PAYMENT">
    <input type="hidden" name="Vendor" value="******************">
    <input type="hidden" id= "Crypt" name="Crypt" value="">
    <input id="payment_proceed" value="Proceed To Payment" class="mws-button green" type="button">
</form>

JS:

$("#payment_proceed").click(function() {
        $.post('/includes/cart/ajax/sagepay.php', { //This script just echos the base64 "Crypt" string required by sagepay.
                "cart_id": <?php echo $cart_id; ?>,
                "title": $("#title").val(),
                "first_name": $("#first_name").val(),
                "last_name": $("#last_name").val(),
                "telephone": $("#telephone").val(),
                "email": $("#email").val(),
                "line1": $("#line1").val(),
                "line2": $("#line2").val(),
                "line3": $("#line3").val(),
                "town": $("#town").val(),
                "county": $("#county").val(),
                "postcode": $("#postcode").val()
            },
            function(crypt){              
                $("#Crypt").val(crypt); //The base64 "Crypt" string is inserted in to the hidden form field here.
                alert($("#Crypt").val()); //I just put this here to make sure it had received the "Crypt" string.
                $("#sagepay_form").submit(); //This doesn't work...
            });
    });

I have no idea why $("#sagepay_form").submit(); does not submit the form. My only guess is it's because the "action" url is a remote server. If this is the case, does anyone know how I can go about what I am trying to achieve?

Thanks, any help would be greatly appreciated.