Time for some old-skool gaming now and get back to the original Borderlands.
C3K3J-939JK-KJ6J9-3JJBB-655ST - expires 4 May, 3:00pm
CBKTB-X3ZJK-5JX3S-33T3J-55F5F - expires 5 May, 4:00pm
You don't need to own the game to redeem the keys. These are not codes to unlock the game, they're only for unlocking chests to receive special loot.
Direct link to redeem at SHiFT is https://shift.gearboxsoftware.com/rewards, or redeem through your respective platform (multiple redemptions possible this way).
Thank you dealbot for the pointer.