Remember that you have enlisted their services. In other words, they are working for you. Their work is an investment in which you have put money, and you deserve good returns. They should give you a free title page, formatting, outline, and bibliography.
Religious professionals used cave paintings to bring good luck to the hunt or to cure the sick and injured. Fast forward to Middle Ages, when scribes used to write because there were a few literate people.
The regular chatting with a writer is very important as you are able to control the writing process in this way. Ideally, such a service should have a real-time chat for customers and writers. So, make sure of it in advance.
You probably would like to read the writing before paying for it, wouldn’t you? And if you are unsatisfied with the work, you have a right to ask to edit it. Clients are the priority in such services, so you should be also offered a front page, outline, reference list and structuring at no charge.
Some of those sites employ non-native speakers and writers without university degrees and that might impact on the quality of their works. To avoid this, read as much custom reviews as possible. You can also check the requirements for applying on the service as a freelance writer.