So you have a very specific target audience built into your store. Are you advertising by following people on Instagram?
I believe that this will not work if you're not running paid ads to start, because with paid ads you're actually able to target people who watch anime AND love coffee.
And like Iknowsomthings said, people are often hesitant to buy food/drink online. Try advertising coffee accessories such as coffee cups.
Anime is also very broad. You're going to want to pick a specific anime and create a POD product like a coffee mug with a famous quote from that character.
From there, you can us FB ads.
What we're looking at is people interested in "coffee" on Facebook. This audience is pretty large and should be narrowed down more with more specific coffee terms like "I love coffee", or "starbucks", etc.
The audience is mostly women. You could also only target men because that is still around 20 million people.
Looking at a more specific anime like "Fullmetal Alchemist", we can see that a majority of the viewers are men. This presents an interesting situation where a majority of the people who like coffee on Facebook are female, and the majority of people who like Fullmetal Alchemist are male. The gender ratio is almost perfectly reversed.
Another thing to think about is that you're going to want to target people who are 25+ because they're often more willing to spend money online. The thing with most of these animes I've been looking at on Facebook Insights is that there audiences are often majority >25. This isn't too much of a problem, but it does cut down your ad audience if you only target >=25.
My main point is that you need to figure out who your target consumer is! Right now if you're not running advertisements, it seems like you're just throwing a bunch of people past your product and hoping that one buys.
These super niche-specific products have potential because you're usually not competing against anyone else, and so the people you're advertising to have never seen any coffee+anime specific products. A major downside is that there is no room for expansion after you reach a certain point, although I do see at least $100+ in sales are possible each day if you approach this correctly.
Keep in mind that you should also have an advertising budget and that it will cost you money.
Good luck!