I can't get the exact price of $56 like you OP, but I got info to help you out.

So we know about the Prime 20% discount, but Amazon also has a Pre-Order Price Guarantee

Amazon.com's price for not-yet-released items sometimes changes between the time the item is listed for sale and the time it is released and shipped.

Whenever you pre-order a video game, the price we charge when we ship it to you will be the lowest price offered by Amazon.com, including Prime savings when applicable, between the time you placed your order and the end of the day of the release date.

A pre-order price guarantee message will appear on your Order Summary during the checkout process.

If you use 1-Click ordering, pre-order price protection will be automatically applied.

The order summary in Your Account will reflect the lowest price typically within 24-72 hours following the release date.

Since you pre-ordered in May, any price drop that might have happened, you automatically get locked into the lowest price, even it goes back up by the time it ships. You still receive the 20% discount off the lowest price too.

Let's check for price drops.

http://camelcamelcamel.com/Call-Duty-Infinite-Warfare-Xbox-Legacy/product/B01EZA0DEI

Price dropped to $72 on August 28th. This is now your guaranteed price before 20% discount unless it drops lower.

$72-20% = $57.60

So somewhere they are taking another $1.60 off your price that I can't find. Amazon wont do anything about a $2 mistake on their part if there even is a mistake. You'll be fine OP.