I disagree with the prevailing sentiment here. Availability during standard office hours is an expected part of any free-lancing gig. Nobody expects you to be available for the full 8 hours, but it is always expected that you will be available during a predetermined part of the day. Just ask any contractor in India or Poland doing a contract for a North American company.
In other words, you should say something like "$X for office hours availability. I'll give you a discount if you drop that requirement".
Charging separately for that will raise flags at the client's end. Also, many enterprises find it easier to deal with higher bundled rates than with more complicated structures.
That being said, what kind of availability are they asking for? AFAICT, the standard is for "same day" response. If they send a question in the morning, a response of some kind is expected in the afternoon, and vice versa. Much more than that justifies higher rates, IMO.
In other words, you should say something like "$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.