Princess Luna has never shown anyone her box before. It's not that she didn't want to, it was that she was nervous that people would make fun of her. At a special meeting between her, Celestia, and Twilight Sparkle, she mentioned her box. It was only a quick second, and there was no gesture towards her having a special box. But Twilight new, deep down, what Luna was hiding. So when the two were alone while Celestia went to the restroom, Twilight asked to see it. Which wasn't a big deal.

Until it was.

*I wrote this off the top of my head when Spike was shopping in the square. This probably isn't that good, so try not to downvote too much, ok? Hey, I was bored!*