There is nothing needed by you to get the add to cart from form working on the IPN page. When you put this server behavior on the page it will start out by checking the PayPal transaction to ensure that it is valid. Next it will loop through the posted values to repopulate the cart based on this posted info.
Once that is complete this IPN page will have access to the cart with the items in it that the user purchased. You can then add in a Universal Email server behavior to this page. In the body of the message you will insert a new cart display manager, from the display manager interface you will choose the email friendly cart display. For the UE server behavior you can trigger on any form post but you will need to make sure that you have the UE server behavior on the page after the 'get contents from form' server behavior since it relies on the cart being populated to send the email correctly.
Since this page is never visited directly by the user what will happen after a transaction is that PayPal will send a response to your IPN page. This response will then be evaluated by the 'get contents from form' server behavior which will repopulate the cart. Then the UE server behavior occurs and sends the user an email with the receipt info.
You may also want to add in an update server behavior to ensure that the status of the order in the db is updated to be successful.
