The postage on a wedding invitation varies quite a bit depending on the invite. There can be a huge difference in paper weight, number of inserts and envelopes, invitation shape, ribbon detailing, crystal gem stones, etc. All of it makes a difference in the cost of sending it to your guests. The one thing that does not differ from invitation to invitation is it is a huge mishap to have the wrong postage!
Make sure you take all of your invitations to the post office to be weighted for correct postage value before you put them in the mail. This will relieve a huge amount of stress about whether or not your guest received their invitation!
Invitation in picture is in the Yummies line by Mmm Paper.
Stamp photo from US postal service website.