I'm going have at least a dozen centerpieces for my small-ish wedding. I'm inviting a few people from out of town, and provided that they are able to make it, I had the idea of giving away a few of the centerpieces to those who traveled far (defined by out-of-state and/or the need to stay overnight in this area in order to make it).
So now they have to figure out how to get the centerpieces in their luggage? I'm assuming they're not flowers (i.e., water in the vases).
I think you should skip this idea for that reason. Even without the water, it's annoying to have to pack something home.
I do think people might feel that it's discriminatory, but -any- reason to give the centerpieces away is a bit discriminatory.
I believe that the way to thank people for going to the extra work of traveling to your wedding is to mention it to them in person or in the thank-you note for their gift (or both). And to allow that gesture they've made to affect your relationship
with them (and for you to bestir yourself to keep a close relationship
with them despite the distance, perhaps traveling in order to be at their special event, or to simply visit them).
Re: the RSVP information.
It is actually *standard* to have this info on the invitation itself. Lots of etiquette books show how.
As does this (the royal wedding invite):http://hipink.files.wordpress.com/2011/05/kate-will-royal-wedding-invitation.gif?w=480
It goes in the lower left corner (dress code goes in the lower right), and it can be any form--phone number, full maining address, email address, website.
If everyone is assumed to know the host's address, it can simply say RSVP or The pleasure of a reply is requested.