I charge a flat day rate for this sort of event. There's not much chance that anything you take at an event like this has the potential to be used in anything other than newsletter, emails, etc. I've done this in the past, and have subsequently been called for other shoots with the company.
My advice is to make sure the images you get to them are all your best effort. i.e. pp, etc.
Don't expect their
PR department to want to do any post processing. I will weed out the junk, mass color correct, sharpen, etc. It might take a while, but if you get more gigs it can be worth the effort.