In other words, you should say something like "$X for office hours availability. I'll give you a discount if you drop that requirement".
Charging separately for that will raise flags at the client's end. Also, many enterprises find it easier to deal with higher bundled rates than with more complicated structures.
That being said, what kind of availability are they asking for? AFAICT, the standard is for "same day" response. If they send a question in the morning, a response of some kind is expected in the afternoon, and vice versa. Much more than that justifies higher rates, IMO.