Hi, I ended up getting a refund from them.
Timeline:
Jan 27th - I bought 1 three month Game Pass code. I used it and my Game Pass account which had expired earlier in the month now showed three months on it.
Jan 31st - I bought 2 more (the limit per purchase). I tried one of these and it gave me an error saying that the trial code is for new customers only.
Jan 31st - Sent an email to cdkeys support about the error. Later that day I got an email back with an apology and stating they were giving me a refund on the purchase.
Feb 1st - Got a full refund.
You could try your own luck as I assumed from comments earlier in the thread that they were stack-able but that wasn't the case for me.