There was a promo code for a game that I wanted, along with expansions. I wasn't sure that the game would run for me (I use Linux, and can never be fully sure that a Windows game will work until I try it), so I bought just the base game and tried it out and it worked fine. Then I went back to buy the expansions but the promo code doesn't work anymore because you can only use it once.

I can't see any reason to do things this way. I think that should be changed.

Please name a game, dlcs, and promo code.
There is a good chance someone might help you : )

Master of Magic. It's a mailing list promo code, but it expires in a few hours anyway. I think it's too late.

That's okay, I just think that this policy is a strange one and should be reconsidered.

Plenty of codes there: https://www.gog.com/forum/general/newsletter_discounts

I doubt all of them, if any, are used.

PQXTXP9NV9WNRP7RV* we are all friends here

*=7

edit: 1hr and 27mins to use it