Error 404

I keep getting an error 404 when trying to update my shopping cart, (ie, deleting an item or changing the quantity). I originally had the WP shopping cart showing at the bottom of the products list but when I kept getting the error I switched to adding a cart page and had that page open up when a product was added to the cart. That works fine until I try to update the cart then I just get the error 404 showing. Does anyone know how I can fix this? I’m using SalesJunction theme in WordPress, if that helps. Thanks!