Add to cart feature on my marketpress & gridmarket theme keeps showing wrong quanty.

Hi, I am totally new to wordpress & markrtpress, and
I've created a website using gridmarket.

The add to cart feature seems buggy. I have 2 problems:

1. When I add 1 item to my cart, the cart will display 2 items instead of 1.

2. When I have items in my cart and move to another page on my site, the items disappear. But when I click back on the page I was on previously, the items reappear in my cart.

Any ideas how to fix this?