For specific rates, you should check USPS (or your local courier). To ensure you have the correct postage, take a single invitation, including any and all enclosures, to your post office or courier to get the final cost. Note, square and oversized invitations and cards require extra postage.
We would hate for your invitations to be returned to you for insufficient postage!
There are also minimum size requirements to mail an item. For example, as per the USPS, you can't mail anything smaller than 3.5"x5". Envelopes and postcards need to be at least that size.