Zen Cart Script Tweak, Rework

I am currently running a script that checks an item as being in stock or out of stock on another webstore, if the product is in stock, the item is able to be clicked on, taking you to the product page of the item in Zen Cart. If out of stock, a pop up window appears informing you that the product is out of stock. There's a big problem with this script in that it can be avoided altogether. The problem is that if going directly to the product page url, you are able to order the product regardless if it is out of stock at the outside webstore. I would need the script to be fixed so that the product page itself checks if the item is in stock or not and returns that result.