It sounds as if you are just doing one yard. The explanations given above were people who have many yards and most clients get 6-8 apps a years of fert, pre ems, etc.
I would charge her the actual cost of fert + your labor charge for spreading it.
I have no idea what size yard she has.
It should be noted that it is not worth it for you. The only way I can see it being worth your time is if you already have the bag on the truck, or she picks up the bag herself and you do it quickly after a mow and blow.