// send non-negative numbers to PayPal Standard
add_filter('gform_paypal_query', 'update_paypal_query', 10, 3);
function update_paypal_query($query_string, $form, $entry){

    parse_str($query_string, $query);

    $items = 0;
    $amounts = array();

    // find out how many items are in the cart
    foreach($query as $key => $value){
        if( strpos($key,"item_name")!==false){
            $items++;
        }
    }

    $maketotal = false;
    $newtotal = 0;

    // find out if any of those items are negative (discounts, coupons)
    for($i=0;$i<=$items;$i++){
        if($query['amount_' . $i] < 0){
            $maketotal = true;
        }
    }
    // if not, send on to paypal in regular format
    if(!$maketotal){
        return '&' . $query_string;

    } else {
        // otherwise,
        for($i=0;$i<=$items;$i++){
            // calculate the grand total
            $newtotal = $newtotal + $query['amount_' . $i];

            // remove the negative items
            $query['item_name_' . $i] = '';
            $query['amount_' . $i] = '';
            $query['quantity_' . $i] = '';
        }
        // make one new item that combines all items into one
        $query['item_name_1'] = 'Registration Total';
        $query['amount_1'] = $newtotal;
        $query['quantity_1'] = 1;

        $query_string = http_build_query($query, '', '&');

        return '&' . $query_string;
    }
}