I think it is ok to sell it.
I would think that in this particular case custom order means made to order-so you obviously charge extra for your time, as you make something that is not for sale in the shop. So it is not like she has commercial rights to this design, unless you gave her those?
But maybe you could also let her know about it? I think I would do that just in case- not for legal reasons, but just out of politeness, and to avoid unnecessary resentments ( because you can never know what she might be thinking, when she suddenly stumble upon it somewhere in searaches ).