I had this problem and then I spend a long time trying to get the answer. My problem was the same, I could not figure out where to edit this. I had a person do most of the website work for me, so I only work from Zen cart. The answer to this problem and I am sure someone else is in the same boat as I was... is to log into your control panel from your host, go into file manager and click until you get to

"includes/languages/english/shopping_cart.php"

Edit away!