How Do You Choose a Book Publishing Service?

I’m preparing my manuscript and researching a book publishing service, but I’m unsure which factors deserve the most attention. What should I compare first editing, formatting, distribution, royalties, contracts, or author support? How can I identify a trustworthy option without relying only on reviews or marketing claims? I’d appreciate serious, honest suggestions from authors who have used publishing services and can share practical experiences, mistakes, or lessons learned.