The Assassin and the Healer - Sarah J. Maas

This was more of a short story than a novella. It give more insight into Celaena and what makes her tick. Also, according to the authors blog, the events in this store wind up impacting the story later on. I can't think of how they impacted anything yet, so I assume it's something coming in the last book.

Also, the only way to own it is to purchase The Assassins Blade, which compiles all 5 novellas in to one book. I already own the 4 original novella separately and didn't want to re-buy them, so I found the novella online here:
You're welcome.

From the authors blog:
"Then there was the fifth, unexpected novella—The Assassin and the Healer. Okay, it’s actually a short story. I wrote it for the 2013 World Book Day in the UK as an exclusive bit of content, and wound up loving it so much—loving Yrene Towers so much—that I wanted to make sure that it was someday available to all TOG readers. (Especially because the events in Healer wind up having a big impact later on.) Yrene is the only character other than Celaena to get her own POV in these novellas (save for a very short scene with Arobynn in Empire). So, make of that what you will."