A fixed position like that will give problems any time the window gets narrower than a certain (very wide) width. If you want it always in the corner of the window no matter where the user scrolls, you might try the lower right, and restyle it to more of a corner triangle (which I have seen plenty of for security icons). That would be less obtrusive.

If you want it to stay in the header (a better idea, I think), you can give #headerWrapper {position: relative;} so that it acts as the positioning reference for the cart box; then you can use position: absolute; for the box location.