Weight: Paper thickness is measured by weight. The higher number you see, in pounds, on the packaging label, the thicker your paper is. This doesn't mean you have to find the thickest paper you can—most card stocks are a great weight for invitations and can hold up to being mailed.
RSVPS: Whether you choose to include a card for your guests to drop in the mail, a phone number to call, an email address to reply to, or a website to visit in order to collect your RSVPs, prepare yourself to have to hunt down more than a few people, regardless of your chosen method. We saved paper by only sending physical RSVP cards to our older guests (i.e., those who do not, and never will, own a computer or a smart phone) because we knew we'd have to chase down responses anyway. So we saved a little bit of money where we felt it might be thrown away otherwise. Here are a few more tips for collecting RSVPs.