Hello,
I just have a look on your website, and there a lots to say, let's make step by step :
- Notice error, isn't enough to create "real" issue situation (most often it's quite impossible to don't have notice error for a complex component like HikaShop), you see this only because your set up Joomla debug mode level too high.
- You have some display issue that leads difficult understanding of your website, to correct this and continue your work more easily I advice you to switch your fontend template for ProtoStar(or any default template), that can even maybe solve some "refresh" feature, like you speak about the add to cart refresh process.
- For your display price issue, check the way you configured your currencies, and more especially how you manage the Format in Locale information part.
Let's see results with these little advices, awaiting news from you.
Regards