+=item amount - The amount of the transaction, as a decimal number. Required.
+
+=item description - A description of the purchase. Required.
+
+=item invoice_number - Your invoice number, if this payment is associated
+with a specific invoice.
+
+=item currency - Currency, specified as an ISO 4217 three-letter code, such
+as USD, CAD, EUR, AUD, DKK, GBP, JPY, NZD, etc.
+
+=item customer_id - The customer number, if any.
+
+=item email - Customer's email address.