I need an advice.

I don’t want to sound boastful, but I’m pretty good at writing stories, and so I got an offer to publish my stories in a small collection of essays. I have money to publish my book, but I'm not sure if it's worth it ... what do you think?

If you want to publish your book, why not do it? If you are confused by the cost of such a service, then you can apply a little enhancement to significantly save on this. I know that here https://www.ls.graphics/free-mockups/book-mockup you will find sample covers for books of different types and formats, so you shouldn't have any problems creating a cover for a book yourself. I think this is a fairly simple task, even for a beginner in this.