Expect a simple invitation with an RSVP card and RSVP envelope printed on card stock to be between 1 oz and 2 oz. USPS prices for basic postage can be found here.
There is upcharge for square invitations, no bend requirement, seals etc. Some 3D elements such as tassels etc. can make the invitation be treated as a package and not a simple letter, which may be subjected to different pricing.
Panel invitations or invitations with multiple inserts that are contained in a folder will likely amount to an ‘invitation package’. Anything that has thickness (greater than 1/4th inch thick) may be treated as a package. Final postage depends on several elements such as weight, thickness, 3D elements such as seals, tassels, manual/automated processing so it is best to assemble one invitation and have the post office assess the cost for you before you carry everything there on the day of mailing.
At Samvadiya, we can create very elaborate invitations that could be forever etched in the memory of the receiver, but we do not hold ourselves hostage to complexity. A simple single invitation with an RSVP card has the same potential to impress just as a full suite ensemble.