Service publishers are akin to vanity publishers in that they will print or publish whatever you have for a price. They will not pass judgment on it and they don’t care if it sells. The difference here is that you know from the get-go that they are selling services, not dreams.
Should you consider a publisher you have to pay? Hard to say, especially as writing gurus start telling self-published authors they need better editing, formatting, cover art and promotion plans, none of which are free.
Melissa Bowerstock tells us that service publishers and vanity publishers aren’t the same because vanity publishers are selling dreams and service publisher are selling services. Before you say, “oh, well that’s just semantics,” consider the fact that service publishers offer a “menu” of services from which you can choose while vanity publishers offer you a package that may or may not include services you don’t want.
It’s worth looking at because self-publishing really isn’t free if you want your book to have a chance of selling well.