Oscommerce Paypal IPN problem

I've been getting the error message "Some required information is missing..." when I use the Paypal IPN payment method using the per item function.

I've managed to find that the page which is sending the info to paypal is storing the form in the following format. Any ideas why I'd be getting the error?

<form name="checkout_confirmation" action="https://www.paypal.com/cgi-bin/webscr" method="post">

<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="item_name_1" value="product 1">
<input type="hidden" name="amount_1" value="8.99">
<input type="hidden" name="tax_1" value="0.00">
<input type="hidden" name="quantity_1" value="1">
<input type="hidden" name="shipping_1" value="0.00">
<input type="hidden" name="item_name_2" value="Product 2">
<input type="hidden" name="amount_2" value="14.95">
<input type="hidden" name="tax_2" value="0.00">
<input type="hidden" name="quantity_2" value="1">
<input type="hidden" name="num_cart_items" value="2">
<input type="hidden" name="amount" value="23.94">
<input type="hidden" name="business" value="info@weirfire.co.uk">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="H_PhoneNumber" value="0123456789">
<input type="hidden" name="first_name">
<input type="hidden" name="last_name">
<input type="hidden" name="address1">
<input type="hidden" name="address2">
<input type="hidden" name="city">
<input type="hidden" name="zip">
<input type="hidden" name="state">
<input type="hidden" name="country">
<input type="hidden" name="email" value="info@weirfire.co.uk">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="invoice">
<input type="hidden" name="custom" value="2">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="notify_url" value="http://www.domain.com/store/ext/modules/payment/paypal_ipn/ipn.php">
<input type="hidden" name="cbt" value="Complete your Order Confirmation">
<input type="hidden" name="return" value="http://www.domain.com/store/checkout_process.php">
<input type="hidden" name="cancel_return" value="http://www.domain.com/store/checkout_payment.php">
<input type="hidden" name="bn" value="osCommerce PayPal IPN v2.1">
<input type="hidden" name="lc" value="GB">
<input type="image" src="includes/languages/espanol/images/buttons/button_confirm_order.gif" border="0" alt="Confirmar Pedido" title=" Confirmar Pedido "></form>

Code (markup):

 

Does paypal give this error or the actual script?

 

have you tried posting this in the oscommerce forums?

 

I've found that the missing information is the name="invoice" part.

When I post it in oscommerce forums people just keep posting their own problems and yours never gets answered.

I dont know why the invoice id isn't getting attached to each post but at least I now know the source of the problem.