Postage Estimate?

You'd be surprised. I thought for sure I would only need one normal stamp, but I had my FH weigh it at the post office just in case and they stated they would still need the more expensive postage. I think part of it is also if you have a larger than normal envelope size or something like that. When in doubt, see if your post office is open so they can weigh it and let you know.

For a domestic piece of mail, one stamp is good for one ounce. You'll probably be fine but I would go to the Post Office weigh them for accuracy to avoid mail getting returned for postage or your guests having to go to the Post Office to pay the additional postage.

Possibly. That sounds similar to ours, and we only needed 1 stamp. However, the only way to tell is to weigh one yourself. If it's over an ounce, you need higher postage.

I was able to to use one regular stamp on outside and one on rsvp inside.

Hmmm....I'm only using a normal stamp, but our's is just a one-sided invitation with a Postcard RSVP card and Details Card

I feel pretty confident that that would be under one ounce, but it definitely would be more expensive/annoying if they were returned. Mail is an essential service, so you should be able to go to the post office. Or, if you have a food scale, you can weigh one at home.