For successful payment process you should use next parameters:

   · username - your username
   · product  - product ID or name
   · action   - use "product" if this product is pre-defined
                use "subscription" if this product is subscription
                use "payment" if this is simple payment transaction
   · price    - price of product, $
   · quantity - quantity
   · period   - subscription rebilling period, days
   · trial    - trial period, days
   · setup    - setup fee, $
   · tax      - tax fee, $
   · shipping - shipping fee, $
   · ureturn  - return URL
   · unotify  - notify URL
   · ucancel  - cancel URL
   · comments - product description

After successful payment process system will forward buyer to your site
and some parameters will back to your script by the POST method:

   · action   - type of transaction (product/subscription/payment)
   · pid      - internal product ID
   · pname    - product name
   · buyer    - buyer username
   · total    - total amount
   · quantity - quantity
   · comments - buyer notes
   · referer  - system referer URL ()

You can get this parameters by the global POST variable,
e.g. $_POST[--VARIABLE-NAME--]...