How to get discounts on books from other subscription?

I heard Vox mention that if you are a subscriber to one subscription (either Castalia Library or Castalia History) you could get a discount when buying single books from the other? How is this done exactly? A discount code? I have a Castalia History Subscription and would like to buy some individual books from Castalia Library in the future.

I think if you are a subscriber, then you have a discount code that you can use to buy other books either not in the current subscription or previous books you may have missed before subscribing assuming the particular book/books aren't sold out. It should have come with the e-mail thanking you for subscribing. Mine was described as a library coupon.

Thanks, I searched me email but cannot find any such code. Maybe I will have to email someone about this.