You can either customize the thank you page on E-junkie, or redirect user to your own thank you page on your website. Those are 2 separate things.
If you want to use an iframe to show E-junkie thank you page (which contains download links etc.) on your thank you page, you need to use the txn_id that we pass to your thank you page using GET parameters.
If you are not familiar with GET, you will need a web developer to do this advanced custom setup.
To test the product, Please see "testing" section in our help page (http://www.e-junkie.com/ej/help.php).