Hi,
If you look at the URL of that page in the address bar, you can see twice the domain name in the URL.
That indicate that somewhere in HikaShop or your merchant account, you've configured a redirection URL without adding the http:// before the domain name. Because of that, the system thinks that you've given it a relative URL on your website and prepend it with the URL of your website.
So you need to find the "redirect"/"return"/etc URL setting where you've done that in order to fix the URL and it should then work properly.
Regarding the optimization of your checkout, I would recommend to turn off the "checkout legacy" setting of the HikaShop configuration in order to use the new checkout of HikaShop 3. It works much better than the old one you're using and should help with the "next/finish" buttons.