Amazon payment gateway

Amazon has different payment products. Checkout by Amazon is their regular product, but it works best for products that you send by mail. SimplePay is probably best for electronic products and services that I discovered too late. Make sure you subscribe to the correct thing. :)

Here's the PHP code for the Pay Now button for a single item using the POST form:

// Key from Amazon
$merchant_id = 'your_id';
$aws_access_key_id = 'your_access_key'; 
$aws_secret_access_key = 'your_secret_access_key';

// Set up cart
$form['aws_access_key_id'] = $aws_access_key_id;
$form['currency_code'] = 'USD';
$form['item_merchant_id_1'] = $merchant_id;
$form['item_price_1'] = $price;
$form['item_quantity_1'] = $quantity;
$form['item_sku_1'] = $sku;
$form['item_title_1'] = $item_name;
ksort($form);

// Encode order as string and calculate signature
$order = '';
foreach ($form as $key => $value) {
  $order .= $key . "=" . rawurlencode($value) . "&";
}
$form['merchant_signature'] = base64_encode(hash_hmac('sha1', $order, $aws_secret_access_key, true));

// Return string with Amazon javascript and HTML form
// Assumes you already have jQuery loaded elsewhere on page
// URL link to live site, not sandbox!
$amazon_order_html = 
  '<script type="text/javascript" src="https://images-na.ssl-images-amazon.com/images/G/01/cba/js/widget/widget.js"></script>
  <form method="post" action="https://payments.amazon.com/checkout/' . $merchant_id . '">';
foreach ( $form as $key => $value ) {   
  $amazon_order_html .= '<input type="hidden" name="' . $key . '" value="' . $value . '" />';
}
$amazon_order_html .= '<input alt="Checkout with Amazon Payments" src="https://payments.amazon.com/gp/cba/button?ie=UTF8&color=orange&background=white&cartOwnerId=' . $merchant_id . '&size=large" type="image"></form>';

return $amazon_order_html;