aaaa I'm sorry, but you'll unfortunately have to purchase the DLC again ^^; I can't hand out download links for accidental payments (or for those who try to pay via external methods that aren't available on Itch), as this would be extremely unfair for the folks who've asked for one in the past and have been rejected. It'd also be very tedious and time-consuming task for me to manage by myself.
If you'd like a refund, please contact Itch Support within 7 days of your purchase!! I appreciate your support nonetheless! ^^
Edit: Just to be safe, I've also deleted the screenshot you've sent in the comment below. Please be careful and don't share things like your receipt number online, as there are malicious people out there who'll steal it and claim it as theirs!