Since I don’t play Borderlands I have no clue what shift codes are. Are they a one time use or can anyone use them? If so, enjoy (and post others here).
Redeem using the Social Menu in-game if the rewards site says the code if for another platform.
This article lists a whole bunch of VIP codes that can be redeemed for points.
The second link takes you to the site for redeeming codes / claiming rewards.
Points mean prizes.
If you get enough points you can even buy some weapons (4,000 each).
If you are feeling particularly frisky, there is a small executable written by a Redditor which redeems the VIP Codes for you (there are about 40 odd), up to you if your laziness or mistrust wins out there.
Lastly, there is a Sci-Fi September sale in the Microsoft Store, which if you buy something, nets you 10,000 rewards points for Borderlands.
Another source: @dgShiftCodes on Twitter lists out a few that worked for me on XBox. Same ones listed in the original thread, but someplace to keep an eye on…