Hi,
The IPN URL and the Auto Return URL are two different things.
The IPN is for PayPal to notify HikaShop that the payment has been made and it should be
http://mydomain.com/index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=paypal&tmpl=component&lang=en
Actually, you should not need to set it in your PayPal account since the IPN URL is passed to PayPal by HikaShop on every payment redirect, at the end of the checkout.
The return URL is the URL where the user will go back on your website after the PayPal payment has been made. HikaShop also passes it to PayPal at the end of the checkout and it is:
https://www.mydomain.com/index.php?option=com_hikashop&ctrl=checkout&task=after_end&Itemid=142
(the Itemid parameter will vary based on how your website is configured). However, in that case, it is a manual process and the user has to click on the "return to the website" link after the payment to go back there. So the auto return option of PayPal allows you to automate that, but that requires that you give your return URL yourself to PayPal. Just make sure that you give the return URL and not the IPN URL.