Hi,
Yes, HikaShop is indeed setup to display a thank you message by default.
As Jerome said, you can change that with a small code change, but it doesn't matter much. It's more about a user point of view than anything.
What's wrong is not that the amount of the order remains at 0 after the confirmation, the problem is that the amount shouldn't be 0 in the first place.
I've actually never seen such issue so I'm quite surprised by such issue. Could you provide a screenshot of the order details page (when you edit the order in the backend) so that we can see if you're missing other things in the order ?
Also, could you try another payment with both the error reporting and debug mode options of the joomla configuration turned on ? Do you get any error on your website on the checkout or before the redirection to worldpay ?