QB is really not set up for that kind of thing, the best you could do is create a service item with that statement in the description and use it as the last item for a zero sales price.
But there is no way for the customer to say, Ok give me 6 batteries and have it calculate the additional cost. If it were me, I do would that kind of thing on a paper estimate and when signed off by the customer, then enter it in QB.
QB is accounting and not really suited to work flow type things
