Lots of ways to get from Word to HTML. Calibre, for example, can convert DOCX files to HTML but the code is very difficult to modify after conversion unless your familiar with HTML. It is free, however, whereas Word to Html is not, and I think it's a monthly, not just a one-time fee?
The link I provided is 100% free. One of my projects is an HTML interactive fiction, and part of my process of getting the text out of Scrivener and into a finished webpage is putting it into wordtohtml.net. Calibre, meanwhile, is something that you have to install. What I linked is a website, so it's very quick to use.
If you're seeing things telling you to pay, then you must not be using an adblocker, because I've never seen (or even noticed) such a thing.
Another possibility for the OP is to upload the story to Smashwords. Good distribution and unless they've changed their options, you can offer it for $00.00. Just a thought.
That's good for an eBook. But why would you "sell" a short story as an eBook? Sticking it on a webpage is a lot quicker/easier for everyone involved. Frankly, if I had to sign up for a Smashwords account, download the Smashwords app to my phone, download the eBook, and then read it and it was only a 3,000 word short story, I'd be pretty pissed, because it would be such a colossal waste of my time. Why couldn't the author just stick it on their own website? Or even AO3 for that matter? I wouldn't need to download anything or sign up for accounts. It's just text, the actual size of it is small, so there is no reason it has to be packaged up as a PDF, ePub or MOBI.