The Commission Process - Custom Design Work?

Hello again, fellow maskers!  I'm back after my last question about packing masks with yet another question.  I've finally gotten my first inquiry as far as making a custom mask...and I'm finding myself a little awkward in the process!  I am wondering if I'm doing things in the most effective manner?

I'm kind of stuck at the very beginning of the commission process, right now!  If I'm making a custom design for someone, should I be paid before I show them my design drafts for their particular mask?  I'm afraid that if I do the design thumbnails for free that a customer might stop there, take the designs, and either give them to another artist to make OR make the designs, themselves.

Is this mistrust unfounded? Am I being paranoid? Should I just offer free quick thumbnails of my custom mask designs to give a customer a better idea of what they're buying?

Along the same lines, should I be charging more for custom masks to cover the fact that I'm doing the extra work of thumbnails and such?

Do share your advice, if you have any!