Hi,
I checked on your link and I couldn't see the problem.
I am redirected back to the product page when I click on the add to cart buttons on your product pages.
Note that I see that you have the "add to cart legacy" setting activated on your HikaShop configuration and thus you're using the old add to cart system of HikaShop. I would recommend switching to the new add to cart system which is better.
With the new add to cart system (if you disable that setting), you'll be able to configure the behavior of the add to cart thanks to the "HikaShop cart notification" plugin via the Joomla plugins manager.