I would remind them of the EU's distance selling regulations which provide a 14-day cooling off period. I think they have that tick box which is meant to imply you waive those rights but very often in these situations that isn't the case as courts tend to take a dim view of those clauses. This might not make them budge on the basis that you won't take them to court over it because it isn't worth it, but it might make them relent. They're basically being a bit shortsighted taking the stance they have because it's a great way to dissuade a customer from buying from them in the future.
I'm assuming you're in the EU as you paid in euros and that you haven't downloaded the content.
How to claim the 2-year guarantee for EU purchases, get a repair, replacement or refund, how to cancel orders made outside shops (online, by phone, mail order).
europa.eu
And if they claim you're past the deadline, remind them you asked to do a return in the first place and that if you didn't mention the DSR, that isn't your fault.