Redeem a code?

So I've been thinking about buying 5e phb and xge on the roll20 marketplace.  I'm hesitant because I own the physical copies, and don't want to spend so much money for digital.  I just noticed the redeem code on the marketplace, where do you get codes and do you get the books for significantly less with said codes?  I really can't justify paying more than $15 each (they're digital, and I already own the actual books).

Redeem Codes is used for Marketplace Artists to have giveaways for contests or at conventions, etc.  You won't find one for the WotC products. There are no discounts on them for owning the physical copy. 

Also, they are not just digital versions of the books, like a pdf. They are completely transformed to allow for Roll20 integration and automation. That's a significant amount of work.