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.
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.
You have to be certain of the guarantees offered by the custom writing company you have selected. Do they have an on-time delivery guarantee? Do they use up-to-date sources? Do they guarantee the confidentiality or that they will not resell your paper? You need to ensure that the company whose services you are hiring guarantee all these aspects.
Some have opined that students go for these services because they are lazy. This is partly true, but the biggest percentage that goes for these services are students who are trying to beat serious deadlines or have a deficit in their writing skills.
Every person has a writing style that most professors are able to tell because they have marked your work for long. As such, the writer that you settle for must be able to match the essay with your writing style.