Here's the relevant code:
var expires = "";
var date = new Date();
expires = "; expires="+date.toGMTString();
expires = "";
document.cookie = name+"="+value+expires+"; path=/";
Full code: http://kisildalur.is/web/scripts/cart.js
You can see it live if you go to http://kisildalur.is/?p=1&id=7 click on "Kaupa" (Icelandic for Buy) on any item. You will see a nice simple animation once you do. Once an item has been added, a cookie is created but in my Opera 10.53 the cookie does not exist. Browsing any other pages will result in the cart being empty.
What am I doing wrong? :S Works for every other major browser except Opera.