Users' login details are lost after redirected from WorldPay

Question

I am developing an e-Commerce site and I am using login control for users to login.
Once they login they can go to search page and search for products and go ahead with their purchase.
To make the payment I am using WorldPay. After the payment is completed, I want to redirect the users to
their purchased cart. But, the users are automatically logged out. They have to login again. I WANT TO KEEP THE USERS LOGIN DETAILS AND KEEP THEM LOGGED IN EVEN AFTER THE PURCHASE, UNTIL THEY CLICK ON Logout button.
I have given a web.config file inside Users folder, for extra securit and it has the following lines of code:-

  <location path="PurchasedCart.aspx">
    <system.web>
      <authorization>
        <allow roles="Administrator,RegisteredUsers" />
        <deny users="*" />
      </authorization>
    </system.web>
  </location>

Sometimes when I remove my web.config from my Users folder it is working.
But, I use this extra web.config for extra security.
How can I make the user keeps logged in even after the payment is completed
(when he is redirected from the WorldPay as well)?