Postage for rsvp to the States!?

I would leave the postage blank. unless you live close to the border. then pop over to the US and get stamps there.
they usually won't get back to you from the US with Canadian stamps.

I live close to the US border so I'll probably pop over one day and pick up stamps from the nearby US post office. Otherwise I wouldn't worry about getting a stamp and maybe for those in the US write a small note saying to call or email their response in lieu of sending it. I'm sure your American peeps will understand

We left the stamps off international RSVPs. You'll be find if you do that

For international RSVP returns your better off to leave those few up to the guests to put a stamp on to mail back. We decided to go with online RSVPs but when I was looking all paper cards I did see a guideline that said for international invites it is best to leave it up to the guests to stamp the RSVPs when they mail it back.

Yes you have to use US postage to mail something in the US even back to Canada.
I had RSVP envelopes sent to Europe but I asked them to kindly do it online. I also wrote a letter to each person in Europe because it was overdue anyhow, and then said that I was sorry I couldn't include the stamp with their envelope and I hope that I'll see them soon. If you know people that go to the US or you're close to the Border yourself just pop over and get a stamp; post office will know how many you need.

Its the same. I accidentally sent the same postage as i did for Canada (forgetting shes in the states), i looked online and it says Canada and states are the same postage. (my stamps had no denomination)

EDIT: Oh i misread coming from the states... not sure...

The website might have some information based on what you may want to know. That could help.

Maybe call the post office in the states? I guess it's kinda hard for them to know without seeing the size of it, but you think theyd have some sort of system to know! I'm surprised that Canada post wasnt sure!!