Price protection:
Not for an Xbox it would seem.
http://www.amazon.com/gp/help/customer/display.html?nodeId=201133150
Amazon.com will price match eligible purchases of televisions and cell
phones with select other retailers, including those who sell their
items on our website. For all other items, Amazon.com doesn't offer
price matching.
(emphasis mine)
Also don't forget, it is very easy to purchase items from amazon.com that are NOT sold or shipped by amazon. The policies for those retails are their own.
You could return it if you qualify for that. Log in to your amazon account and look in your account area to see how to return item.
EDIT
One more thing brought up in the comments: you don't get anything if you don't ask. It sure wouldn't hurt to call or email the customer service team and see if they have a solution. Be polite and patient and see what Amazon is willing to do to keep you as a happy customer.
Add to cart pricing:
Walmart and Best Buy are both doing that "Add to Cart to see our
special price" thing. Is this to prevent other retailers from matching
their price?
I believe this is because the manufacturer has restrictions about how a retailer may advertise the price. It is called the minimum advertised price. Sony sticks in my mind as a brand that consistently did this. To protect their brand and retail "channel", stores couldn't advertise a rock bottom price on a Sony stereo. Sony cannot ultimately dictate the final price, but they can dictate the list or "retail" price. So deep discounting retailers just waited to show the price in the shopping cart to get around the manufacturer's restriction.
I assume the goal is to keep a bulk of retailers happy with the manufacturer. (aka - channel management.) If Microsoft has 10 retailers selling their goods, and only 1 of those 10 is a deep discounter, then the other 9 are much happier to stock Microsoft goods knowing they don't really have to compete on price.