the way to go about it if you really wanted to would be to grant usage rights to them (allowing the photos to be used for certain things, a certain number of times, for a certain period of time, etc), and then set a price for that usage.
the other option is to sell ownership of the photo completely to the purchaser, which means you dont own the photo you took any more and need their permission go use it, and you'd generally set a much higher price for this.
if you gave them the pictures with no contract then you still own them and still control usage, and you could pull the plug at any time.
all things considered for a simple shot i agree its not terrible to just ask for a discount on parts or a free part or something. if they are looking for more high end product photos, point them in my direction
