Hi,
What to use will depend on what you want to do with the feedback.
What you can do for example is use such testimonial extension:
www.joomunited.com/joomla-demo//my-testi...ls/testimonials-list
And in the email notifications, that you can edit via the menu System>Emails of HikaShop, you can just add the URL to the testimonial form for the customer and some text around it to explain to the customer.
Then, that extension allows you to display the testimonials in a module so that other customers can see them.
You can find other extensions doing the same thing on the Joomla extensions directory.