If that code is working, it's worth giving them a nudge and asking for them to honour it. I had a similar situation a couple years back where I missed the code for an extra 5% off, and they refunded me the difference before the chair even arrived. Definitely worth a punt. If that code stops working again, usually the bigger Twitch streamers will have a code for 5% off kicking around at this time of year. Have a look for one of those.