Hi,
Where is that "buy" button on your website?
I went to index.php/hikashop-menu-for-module-127 and click on the add to cart button for 2 of your products but I didn't get any redirection. I just saw the notification box.
How do your customers access your checkout ?
I would recommend to configure the HikaShop cart notification plugin via the Joomla plugins manager so that it redirects automatically, or to add a popup with buttons so that the customer can choose what he wants to do.
I don't know how to reproduce the error you're talking about.