[Marta]

It is desirable at least to test customer for seriousness somebody speaks about pre-payment, even though the cost of materials.
  If a customer wants is you, then he would never argue.
  And if you will, it means that the risk is great that he just kidding, not ready to trust your good taste, experience and abilities - better and does not communicate (not to engage in self-deception).
   Especially if ordered, the work is not quite in line with the normal activity of the artist.