Thanks for your feedback! We plan to have a few small things like the menu behavior and a couple of other minor design tweaks done soon. Thanks for pointing out one we missed.
Regarding the comparison tool, that is native HikaShop (business edition), with a bit of styling customization. We broke out all of the different details about each item (ingredients, usage, warnings, etc.) into HikaShop product custom fields. That allowed us to create the useful comparison layout you see today.
Regarding the compare button, I agree it's not intuitive that you should scroll to the top for it, but right now that's how it works in HikaShop. Perhaps I can have our next template contractor do something special to make that button "sticky" or have it float on the side. Something to make it visible immediately to the user no matter where they are on the page, once they have clicked 2 or more items for Comparison.
Thanks again for taking the time to provide feedback and screenshots. Much appreciated!