My immediate thoughts are not really appropriate for an etiquette forum.
2.5 hours early seems really early to me. If the venue needs that much preparation in advance, I think they should have picked another venue, or at least made sure everyone they want to help is in fact available 2.5 hours early, and some time after. If they didn't clear that extra time with you beforehand, and they're expecting you to help, they may be disappointed when you say that due to other activities you have scheduled that day, you'll only be there at most half an hour early, and able to stay after only half an hour (for example).
Wasn't this venue supposed to be someone's house, or am I mixed up? If so, even less reason for needing a group of people working for 2.5 hours to get the place ready. If the house needs a deep cleaning or something, the hostess should either take care of that, or not offer up her house.
Again, if they didn't get your okay beforehand to provide a large amount of specific food and serving items, they can't count on you actually doing so. You might not be able to (want to) afford them, or transport them to the venue.
Pretending the shower proper runs from 1pm to 4pm... I would respond with something like, "Wow, sounds like an exciting time! FYI, I have other commitments that day, so I can be at the venue at about 12:30pm, and I have to leave by 4:30pm. I can chip in $25 towards the cost of food, etc.. Do you want cash or is there something specific I can bring for that price?" Or whatever other limits you decide on. If they make plans without your agreement, they can't count on you to do what they want. I think it would be fair to warn them of that ahead of time, though.