Thanks everyone.  I was starting to think I was behind the times or something.  As far as the clarity of the invitation, I'd have thought that putting the special parking rate and location (and the absence of any mention of other transportation) would indicate that getting there would be the attendee's responsiblity.  However, I do believe that some people like to take "the customer/client/donor is ALWAYS right" thinking just a tad too far when trying to wheedle something "extra". 

My personal take on it was that if the prospect if getting oneself there was too onerous, well, as another site I enjoy tends to say,  "It was an invitation, not a summons."