These days the cart has full integration for 2Checkout.com which is extremely easy to configure. Simply go to the appropriate page in the cart setup in Admin, enter your 2Checkout.com ID and hit submit.
The callback script
It is not essential to setup the callback script if you don't want. It's purpose is to return control back to the cart afer someone has paid. By doing so the cart can send the customer an email, track sales via mTracker and display links when you are using the download system.
With version 2 of their gateway set the URL back to the cart to:
Return URL: http://ww#.aitsafe.com/gen/2checkout.php
Where you use your own server # in the URL, for example ww4.aitsafe.com.
URL to this page: https://www.mals-e.com/tpv.php?tp=12