How'd I manage to unlock the Lace skin by buying from the Subeautique? Is that a thing that happens? Whoa.

Yes, the lace skin can be unlocked randomly when you purchase something from the Subeautique, there's even an achievement for unlocking it.

, yeah I got the achievement as well. I just simply wasn't aware you could unlock a skin that way, thought it was only during Fireside.

There is another Skin called Flameo. That is the Fireside one.