https://www.canadapost.ca/cpo/mc/aboutus/news/pr/2013/2013_action_plan.jsf Right now stamps are 63 cents I believe, but they are to go up to 85 cents on March 31st 2014. It doesn't seem like a lot but if you are buying 100 for your invites and 100 for your RSVPs, that's a difference of $126 and $170 totals. My FI and I are planning on stocking up on stamps beforehand because you can still use them after the rates change.
ETA They always used to honour "old" stamps because it didn't have the price on it. Now it looks like they print the price right on the stamp so I don't know if they will still use the older/cheaper stamps or make you buy another stamp to put on to match the current (in the future) rate.