Decent start

Using 1.2. So far I had to re-stylize the few fields included in the plugin. Definitely needs to have options for additional customer fields (name, address). I have not tested the payment page out yet, waiting on SSL.

I added simple feature to pass in fields like amount and item via querystring. I think this is definitely a need feature

$queryAmount = (float)($_GET["amount"]);
        $queryInvoice = (float)($_GET["invoice"]);
        $queryParameters = array(
            'item_name' => $queryInvoice,
            'item_amount' => $queryAmount);

        if ($queryAmount !== "" && !empty($queryAmount)) {
            // echo '<pre>'.print_r( 'query Params: ' . $queryParameters, true ).'</pre>';
            // Extract querystring args
            extract(shortcode_atts(array(
                'item_name' => 'item_name',
                'item_amount' => 'item_amount',
                'url' => '',
                'button_text' => 'Buy Now'), $queryParameters));
        } else {
            //echo '<pre>'.print_r( 'shortcode' . $atts, true ).'</pre>';
            // Extract shortcode args
            extract(shortcode_atts(array(
                'item_name' => 'item_name',
                'item_amount' => 'item_amount',
                'url' => '',
                'button_text' => 'Buy Now',), $atts));
        }