Add to Cart buttons broken

I need some help.

Our website currently is having problems with the "Add to Cart" buttons from within a product page.

We make decals and the two main categories we have are "Customized Decals" and "Non-Custom Decals". Oddly enough, the "add to cart" button works just fine for all products that fall under the "Non-Custom Decals" category, but ALL decals under the "Customized Decals" category have some weird broken link when you click on the "Add to Cart" button from within the individual product page.

For the customized decals there are many required options to fill in before you can add to cart, and even if those are all fulfilled it still won't let you add to the cart. The problem is not in forgetting to fill in a field with an asterisk, it's the link within the "Add to Cart" button that is either broken or has something else affecting it.

Can someone PLEASE just take a look at our website and try to do a test with adding one of the customized products to the cart? Then you will see what I mean.

We currently are running Opencart 1.5.3.1.
Months ago we did an upgrade from 1.5.1.3 but as far as we know this is a fairly recent development, but we 're having a hard time pinning down what is causing the problem.

We have not added any new mods recently or made any major settings changes. I'm thinking it's something within the code, but need someone to take a look at the website and see if they have any pointers.

Our website is ShowItOff Decals

Click on the tab in the menu bar that says "Customized Decals" and select a decal to test.

Thanks!