How should I word my graduation invitations? Refreshments provided but they pay for dinner if they order?

My mom is throwing me a graduation party at a local center that has a restaurant in the bottom. We are renting a boardroom and will have a cake, punch, and snacks. You are able to order from the restaurant and eat also. Most of the people attending know how the place works and have been to parties where they order their on food. I am concerend about guests that do not though. We started to make it informal and call everyone and say drop by and have a piece of cake, but I would like to send out invitations. I just want everyone to know how it works so I am concerned about wording, this is what I have so far:

Please join us for refreshments to celebrate the GRADUATION of
NAME
Saturday, April 21, 2007
6:30 pm
PLACE
RSVP PHONE #
*dinner may be ordered if you choose

Does that sound like they would expect to pay for their own food and is it rude?
I would just stick w the finger foods, but i KNOW many people will be ordering their own food regardless.
What should I do?