Need help with promotion

With promotions like this, your only option is to contact support, so they can help you further, their number can be found here

http://support.xbox.com/contact-us?isChatCallAvailable=false

That was part of last week's Deals with Gold which ended. Deals with Gold change every Tuesday so unfortunately there would be no way to get the sale price again unless it goes on sale again.

Hello Afro Simian,

We see that your registration date is the same as the purchase date you describe, so you should be eligible for this promotion. While registration takes two days to complete, it will date your registration date as the initial date of your registration request.
However, keep in mind that it will take up to 15 days past your shipping date to receive your Credits as pending.

https://rewards.xbox.com/console-upgrade-offer/

Link for further proof. All that is required is for me to own an Xbox 360, which I do.

That offer is only available to select Rewards members. So you would have had to be a Rewards member before the purchase in order to be eligible for the offer.