I had a situation where something went wrong during the payment process. I decided to contact GOG Support and guess what! They gave me another code! So this is not a huge problem. At least not in normal circumstances when the Support isn't overloaded by work. But I'm sure the answer would be positive even when the discount would already ended.

That just requires unnecessary additional work. Reduce the price in the shop temporarily, at least after one has clicked on the newsletter-link ...... Then, even other games and/or DLCs etc. could be added too and it doesn't have to be a separate purchase.