Eneba has xbox one key for RDR ultimate edition from Argentina for 11.77 USD. Follow the below instructions to redeem for US.
Step One: Purchase the key in the link at the bottom. (Likely you will have to click +9 other offers to get the correct price.)
Step Two: Connect to any VPN service that has servers available in Argentina.
Step Three: Sign into redeem.Microsoft.com and enter your key.
(After successfully redeeming don’t forget to disconnect.)
For VPN- I used an app called VPN.lat (available in App Store for iphone) and was able to complete the entire transaction on my phone.
https://www.eneba.com/us/xbox-red…-argentina