title: Payment Merchants with easy IPN
body: <p>I am looking to integrate to my website many payment merchants.
  Now I have integrated PayPal and CCBill because they have easy to use IPN.</p>
  
  <p>I want to send to the merchant data like: amount,1custom variable and the merchant should send to my IPN url the amount and the custom variable.</p>
  
  <p>It would be nice if you can give me some names and links to their IPN documentation</p>
  
  <p>Thanks!</p>