Trying it yourself is free. Don't send out fifty queries!
Typically, the advice from agents is to find the agents that match your story (MSWL = www.manuscriptwishlist.com; another collection is agentquery.com). You may well find 50, but if you've never written or sent a query letter, send it to SIX to start, and see if you get any feedback on your query.
Once you've queried an agent for a book and been rejected, their assistants may screen further queries from you and the agent will never see the additional queries. See if you get feedback or complaints and keep your powder dry, so you can revise and try again with another six. If you get no complaints and no hits, send out a dozen.
You can also Google for writing a query letter. There is a lot of advice out there. Here is a List of 23 book query letters that worked in different genres.
There are also several claimed "ideal formats", if you are a first-time novelist you should look for one that doesn't demand you give your "credentials" and past writing successes. Every published author had a first successful book query. So it is possible to do without it; just leave it off, or say "This is my first novel;" to start a single line bio. Use any extra space to flesh out your story and intrigue the agent.
Query letters usually only describe the first act, in about 1/3 of a single-spaced page. (The first act presents the "normal world" and the main problem the MC is confronted with).
Some of these services may be good, I don't know. The problem is the contracts for this kind of stuff is money up front, and quality can be uncertain, especially if you have no idea what a good query letter even looks like, or if the agents they choose really are right for YOUR story. So you might get ripped off, and simultaneously poison the well with bad query letters to agents on your behalf.
You DO need a synopsis anyway, an agent might request it; and you do need a completed work, they may request that. (Only query a completed work!)
But I feel, as a writer, that only I can truly write about and describe the story I wrote; nobody is going to do a better job by reading a synopsis I wrote. If my first act has to be summarized, I will decide what is most important to convey and what is less necessary and can be left out, or fudged for brevity (a common practice in query letters, which agents know may be necessary for a complicated story).
All the research can be done online, I strongly suggest devoting a week or two to learning the art of the query letter for yourself. Even if you eventually decide to use a service, you will at least know something about what you are buying!