First off, I'm so sorry that got that review. I would be heartbroken too. Especially since the customer does not let you know at all why she is not satisfied.
Second, like everyone says: luckily you do have gotten many other 5star reviews so you're overal 'status' is still 5stars.
I would definitely reach out to her. If she doesn't respond, put that in your reply to her review, so that everyone that reads your reviews knows that you are a responsible seller who does her best to satisfy her customers.
Lastly, I would up your prices. I think your items look great and as Erin says you're time is worth something. Also, if I were a customer and I would read the review I might think: "well maybe it is indeed poor quality - can I really expect good quality for such a low price?"
"edited"
I just noticed that you charge $2 per letter (which is still low, in my opinion) but it wasn't totally clear to me at first. Maybe you can state that more clearly in your description?
Good luck!