We were able to use standard Forever stamps for ours, so 46 cents/invitation, plus 33 cents for each response postcard (79 cents total). We had:- a cardstock flat invitation, sized 5"x7"- a cardstock RSVP postcard, sized 4.25"x5.5"- an insert that was half a sheet of letter paper, folded- envelope
I did the same. My manager got invites done with a heavier card stock and I think square too, it cost her $1.20 to send each invite
Ours were $0.66 for the invites and a $0.33 for the RSVP
(postcard). The invites were 5X7 cardstock with one 4X6 enclosure card, and one postcard sized RSVP. They make a $0.66 wedding stamp you can buy (so you don't
have to use multiple stamps). My suggestion is to hold off on buying
postage until everything is printed. Take an invite ready to mail to the
post office, have them weigh and purchase accordingly. That way you're
not left with postage you can't use. Also, a suggestion our printer told us was to have the invite weighed at the post office where you intend to mail the invites at. That way you know their scale will match what you expect.