Instant Payment Notification (IPN) is a message service that notifies you of events related to PayPal transactions. You can use it to automate back-office and administrative functions, such as fulfilling orders, tracking customers, and providing status and other information related to a transaction.
I am certified Authorize.net developers, can integrate Authorize.net payment gateway solution with your website. I have integrated many websites with Authorize.net payment gateway using a variety of technologies. Once integration is complete, you would be able to accept credit card and e-check payment through your e-commerce website.