The terms of service are agreed upon between the researcher / client and the expert. Before you make the connection, make sure to get clarity on all the terms. Pricing is just one of those, but also discuss and agree upon timelines and what exactly is included in the service.
There is, for example, a clear difference between language editing and scientific editing. Make sure you ask the expert what is included in their service, for the price they quote.
Peerwith provides the service to experts by helping them connect with researchers. Thus, the experts are the clients of Peerwith. The experts' clients are the researchers. Peerwith does not provide the service to them. Therefore, Peerwith is no party in setting the terms of service.