Re: RSVP courtesy (bit of a rant)
That is very obnoxious behavior, and I hate to say it, but I think your only options are:
(1) only order for the people you are sure are coming (maybe plus a little extra), and only order for the extra people after they come (meaning that they have to wait for their pizza)
(2) order for everyone who might possibly come, and bring a lot of extra pizza home or
(3) call the people who are invited and ask them if they're coming (which you shouldn't have to do, but there it is)
I actually would rank 2 at the bottom of the list. If you don't have the time/inclination to call, I think I would order just for the people I know are coming, plus maybe 1 pizza extra, and then not order any extra pizzas unless/until extra people show up.
(Oh, know there seems to be a consensus on the other thread already, but I thought I'd chime in agreeing: no, you don't need to buy salad, yes, you do need to buy soda. Many places offer pitchers, which might be cheaper.)