Hi,
You're on the right track. You just need to know that when we don't need to add any CSS to an area by default in HikaShop, we don't add any CSS code for it. Still we add the necessary classes/ids so that if you need to style the area, you can add your own CSS.
So just add the CSS code below:
.hikashop_order_subtotal_title { display:none;} to the CSS of HikaShop and that will hide that area. You can for example add it at the end of the file.
Personally, when I need to customize the CSS of something, rather that directly modifying the CSS code already there, I prefer to add my CSS at the end and override what the default CSS already does. That way, when I need to update to a newer version, I can easily get all my custom CSS and add it at the end of the new default CSS and it should work pretty much off the bat, without spending hours finding what I changed where in the old default CSS.
Now we still allow you to directly modify the default CSS as it's easier for most people but if you can do like I said, it will be better for you in the long run, and not only for HikaShop but anything where you need to add/change CSS code.