I've created books and publications starting in the 1980s, when desktop publishing was the start of our self-publishing revolution. I help self-publishers step through these choices. It's a wonderful world that we write in now, full of tools that seem magic while they deliver their power to spread our books into the world.
Many good ebook creating shops, such as ebookit.com, offer a full range of services, including the ability to opt out of any distribution points (like Amazon) you might already be managing yourself. Using an aggregator like this has a potential gap to cross. Any discounting you want to do on Amazon will have to be price-matched at your other outlets (say, Apple Books). Some of the less-popular outlets take a while to revise your pricing.
The service to convert at ebookit.com shows off how much detail you'll have to manage yourself if you use a Jutoh or a Calibre. These are open-source programs, so customer support (if you don't understand the manual) is going to come through a community board (a lot like this one). It can take a while to get an answer for that kind of support, and you have to vet the accuracy. Open-source is a good choice when you're price-conscious. You will still spend when you choose a Calibre: you'll spend your time. Your second book will go faster than the first.
This is where Vellum is a good choice: You want some distribution control, so you submit Vellum-created books yourself to the likes of Apple Books. Ebookit will build this file for you to submit. If you want to make changes to your book, though -- maybe you have back matter that touts a new book -- you'll be in for a revision fee at most service suppliers. This is easier if you control the ebook file; Vellum auto-builds files, after you do all of the detail formatting that a good ebook requires. It's literally tick-boxes for the formats.
As for the details in the conversion, making a quality product -- one where the formatting never comes up in your reviews -- takes close watch. Here are the things that ebookit mentions as aspects of converting a file from Word
Investing to serve your book
When you choose to publish yourself, you sign up for steps like ebook formatting. It's true whether you pay a hybrid service, a formatting-distribution-only supplier, or you choose to do the work yourself. Vellum is less than $250. If you've already done publication layout and design, it can be quick to pick up. KDP will do all of this for you at a dollar cost of $0. You have little control over how the end product looks there, because there's no interative passes for revisions using the free methods. You resubmit. It will be correct, but you get the vanilla formatting. Long ago, I paid BookBaby $249 for my novel's ebook formatting. It took us three passes before the errors were corrected, but at least I had customer service to help manage the 10-day turnaround for each round of changes.
Would you rather be promoting your book, instead of formatting it? Doing promotion is always the author's job, whether you have a book contract or you publish yourself. I have a graphics expert, who does covers, and Asya Blue also creates book interiors. Asya can't write my promotional materials: my job. I have formatted with Vellum: fun, but time-intensive, and probably not my job for the next book. You gotta love the control of using your own tools, though.
Question: Is your book that took months or years of your life to write worth a $250 investment? What's your budget for publication? If you figure that a 200-copy sales scope is all you'll manage, then the $750 you earn feels like not enough to invest in tools or services.
Few self-published books climb above the 500-copy sales range. Those that do ride on the advantage of an author, who's also the publisher, investing in tools and suppliers that make a difference. Can you afford to let the reader think about your story, instead of the ebook formatting you'll do yourself?