You need to decide a few things. One, how will you charge? Will it be by length, or flat fee? Will you want it in advance, either in whole or in part? Two, are you putting any restrictions on requests in terms of content? Three, how will you manage the writing? Will there be drafts and an opportunity for the commissioner to make changes or will it be one shot?
That's probably not complete, but it's a good start, I think.