How much is your time worth? I like MTS's idea of having a service menu. I would think that $30 to cut out the new fabric that they bring you is fair - if you can do it in an hour or under. Anything more complicated than a YBR or TT would have to be more.
ETA: Looked again at your OP. If you are picking fabrics and cutting it could be fabric + cutting + labor which might be $50+. Would they be able to pay that? Now we're getting into why some kits are so high.