Accidently Burned Important Letter!

So I was getting towards the end of the game and I had achieved every combo except for the last two. When I opened the letter with the countdown to explosion, I freaked out and quickly quit the game because I thought it was ending and I hadn't achieved the last two combos. When I re-entered the game I attempted to get those combos (and succeeded) but I wasn't really thinking straight and burned that letter along with everything else (without technically reading it). All I got at the end was another letter saying I had complete the game. I still had my hug coupon, but there was no way to proceed to the real ending I've heard about. I even looked online and learned about the last "error" combo. Even though I tried burning the Broken Magnet, the Jar of Fireflies, the Toy Exterminator, and the Fashionable Sunglasses together, nothing happened at all! I've tried this multiple times.

Was I an idiot and simply ruined the ending for myself, is there a bug, or am I doing something wrong?

I played this game on my recently updated Wii U (not sure if that makes a difference)...

I greatly thank anyone who can help me out!