If you are using Paypal Payments Pro, When you run the checkout wizard, you will be able to configure eCart to send an email receipt.
after the checkout wizard has run, you will need to modify the email receipt that is added to the conform page.
modify the email receipt to include a link to your download page and send the Items ID as a query sting variable. For example:
On your download page, create a recordset to look up the file name, filter the recordsets ID column in the id query string variable to return the file name.
In Digital File pro, set the trigger for the download to occur if the recordset is not empty. Set the Folder to the location that contains your downloaded files. clickthe lightning bolt next to the file name and select the file column from your recordset.