I am using ubercart with userpoints. Sometimes, credits from userpoints are larger than item price (for example, item price = $10, shipping = $5, customer pays with 12 points + $3). Paypal (and other gateways) do not accept subtotal of zero or less. Payment would fail, but worse is that customer do not get error message, but a message saying that her order is complete.

I added the below code to the end of function uc_paypal_ec_submit_form_submit in uc_paypal.module (around line 1150) - removes subtotal, shipping and tax lines when subtotal < 0, also checks for success or failure. It seems to be working fine, but should any of the $_SESSION variables be unset or otherwise changed in case of failuer?

if ($subtotal < 0) {
$nvp_request = array(
'METHOD' => 'DoExpressCheckoutPayment',
'TOKEN' => $_SESSION['TOKEN'],
'PAYMENTACTION' => variable_get('uc_paypal_wpp_payment_action', 'Sale'),
'PAYERID' => $_SESSION['PAYERID'],
'AMT' => uc_currency_format($order->order_total, FALSE, FALSE, '.'),
'DESC' => substr($desc, 0, 127),
'INVNUM' => $order->order_id .'-'. time(),
'BUTTONSOURCE' => 'Ubercart_ShoppingCart_EC_US',
'NOTIFYURL' => url('uc_paypal/ipn', NULL, NULL, TRUE),
'CURRENCYCODE' => variable_get('uc_paypal_wpp_currency', 'USD'),
);
}
else {
$nvp_request = array(
'METHOD' => 'DoExpressCheckoutPayment',
'TOKEN' => $_SESSION['TOKEN'],
'PAYMENTACTION' => variable_get('uc_paypal_wpp_payment_action', 'Sale'),
'PAYERID' => $_SESSION['PAYERID'],
'AMT' => uc_currency_format($order->order_total, FALSE, FALSE, '.'),
'DESC' => substr($desc, 0, 127),
'INVNUM' => $order->order_id .'-'. time(),
'BUTTONSOURCE' => 'Ubercart_ShoppingCart_EC_US',
'NOTIFYURL' => url('uc_paypal/ipn', NULL, NULL, TRUE),
'ITEMAMT' => uc_currency_format($subtotal, FALSE, FALSE, '.'),
'SHIPPINGAMT' => uc_currency_format($shipping, FALSE, FALSE, '.'),
'TAXAMT' => uc_currency_format($tax, FALSE, FALSE, '.'),
'CURRENCYCODE' => variable_get('uc_paypal_wpp_currency', 'USD'),
);
}

$nvp_response = uc_paypal_api_request($nvp_request, variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'));

//check for success here
if ($nvp_response['ACK'] == 'Success') {
unset($_SESSION['TOKEN'], $_SESSION['PAYERID']);
$_SESSION['do_complete'] = TRUE;
drupal_goto('cart/checkout/complete');
} else
{
//Do we need to change any of the session varibales here?
drupal_set_message(t('There was an error while processing your PayPal payment. Please try another payment method.'), 'error');
watchdog('uc_paypal', 'Paypal payment failed: ' . $nvp_response['L_LONGMESSAGE0'], WATCHDOG_ERROR);
drupal_goto('cart/checkout/review');
}
}