This might seem pretty obvious but it shouldn't be forgotten during the invitation design process! Each additional card you add to your invitation suite (details card, rehearsal dinner card, rsvp card, etc.) will cause your invitation to weigh more. If your invitation suite is over 1 oz. you will require additional postage. Right now it is an additional $0.24 per ounce.
Shockingly enough, having an "oddly" shaped envelope will cause you to need to pay more for postage. If your invitations are anything other than a rectangle, you will not be able to ship it as a letter. Anything that is rigid, square, or unusually shaped, often jam postal equipment and are more difficult to process, hence the additional cost.
Anything going to another country requires an international postage stamp which is more expensive than a regular forever stamp. Currently (April 2023), international stamps are $1.45 while forever stamps are $0.63.