It's probably this page here:
http://www.masterbuilt.com/shop/index.html
And this is likely happening for some shoppers when they click on the Continue Shopping button in the cart because this is the URL you have in your Preferences -> Hosting Service menu setting labeled '"My Store" URL'.
The default behavior of the shopping cart is to return the shopper to the last store page they were on before entering the cart when they click the Continue Shopping button, however this function relies on the HTTP_REFERER environment variable that web browsers provide so that the shopping cart can determine what page the shopper was on before they entered the cart. Some anti-virus or firewall suites that some shoppers have installed on their computers can block this variable from being available, and in those instances the shopping cart cannot determine what page they came from. So these shoppers will be sent back to the editable '"My Store" URL' location.
The easiest thing to do here would be to change the '"My Store" URL' setting to go to a different page (one without a search box).
You should also be able to remove the search box from the
http://www.masterbuilt.com/shop/index.html page by locating this page in your ShopSite's Pages menu and edit the page contents and uncheck the 'Product Search Field' checkbox, Save Changes, then Publish.
-Loren