The 25 medallion items seem to actually cost 10, and the 20 cost 5. It appears consistent, implying it was more of some sort of typo than an actual bug. I can return the items I received if I wasn’t supposed to error-test!
Fixed! I had a spelling error so you were only charged for one of the items in the set (oops). Thanks for checking that out and reporting it! Feel free to keep the items you got as a bonus for being early to the event.