Hey there :)
Here's some observations on the shopping cart that may help the software folks track down oddities with the system when they're feeling bored :)
1. I had a "choose your own" gift certificate in the cart in one tab, and in another tab added a preorder to my cart. Back on the GC tab, I hit Update Quantities and it took me to the main page for the flip mats instead of updating the quantity. I was not logged in at the time but the cookie was present that identified me. Unfortunately I hadn't actually made a change so I cannot say if the cart would have kept any changes.
2. Note that I then logged in on a different tab to look at my gift certificates.
3. The two flip-mat preorders I added to my cart did not show up as "Ship with my Subscriptions" as they usually do - I had to change them to that. Note I had added these to my cart when I was not logged in.
4. When I did hit the button to ship with my subscriptions, it brought me back to page 1 of the cart, not page 2. It did this both times I updated the shipping status.
5. On page 3, when I changed my payment information, it did *not* bump me back to page 1, it left me on page 3.
Anyway, hope some of that comes in use to someone :)