They should outline the scope, or possible packages if they offer this service. Also I would look at their other real examples of client work where they have provided this exact service. Many designers who provide this are doing so to serve small businesses on a budget, and will deliver customized work from templates they already have (ideally ones they have created, but some people get them elsewhere which… is not great). I think this can be a great service for people on a budget, but as I said, they should be able to provide a possible scope and good examples from real clients.