If you check "Use default Return URL", then your Return URL will be derived from eCommerce and will be like this:
http://YOURSITE.COM/index.php/checkout/success_download/ for digital goods.
If you want to use a different link, uncheck the "Use default Return URL" and enter the your link in the "Special Return URL" field.
Payonline system does not automatically redirect to the Return URL. The user needs to click "Finish" after the payment.