I agree that the transportation issue is your "out."
Say, "Wow, we'd love to have Mary come, but my car will already be full."
(don't even MENTION the option of taking two cars! What a PITA!! No WAY would I do that, even for someone who RSVP'd early.)
DD had her party at a bowling alley last year, and it was fun, but you still have to get there early, set up the room with the cake, the gifts, etc. (they provided food and paper products, but I had to bring the cake).
Now I did actually bring not one but TWO of her friends to this party, but their moms dropped them at our house AND I had room in my car. So it was no big deal. I would NOT have taken a second car just to bring them, though, and I would not have gone and picked them up. I was frazzled enough trying to remember the candles, the matches (yep, forgot those one year. Not good!), the gifts, the camera, the camcorder, etc. etc.
I also understand about the $$ for an extra kid. We had to choose pizza or hotdogs. Each kid got one hot dog or one slice of pizza, a bag of chips, and a drink. The drinks were free refills, but the food was NOT. An extra kid would not have had anything to eat, unless I bought it a la carte from the snack bar. Plus I presume going from 10 to 11, means going up a party tier. (here, there is a flat rate, regardless of how many show up. You can choose a party of 5, 10, 15, etc. If you have 11 or 12, you still pay for 15). Adding "one more" would have been a huge price increase.
Hopefully, the mom won't be able to find other transportation, and you can invite Mary to go bowling with your DD another day.

You can even give her a balloon and goody bag when she comes. I guess I would find some way to have her at the party if the mom can get her there, though. I don't think I would have resorted to inviting from the "B" list untill the dead last minute. The RSVP issues these days are brutal. Seriously.
