'how much does it cost to mail a wedding invitation in a 6 x 9 envelope?'?

the cost depends upon both the dimensions and the weight of the envelope. the weight of it depends upon how heavy the paper is that is used and how many inserts you have, like stamped self-addressed rsvp cards, map to location, hotel info, etc.

we are not able to tell you this. take a sample of the invitation to the post office, already stuffed with everything that is going inside and they can tell you . there will be a different rate for domestic mail and overseas mail, so if any of them are going out of the country, tell the postal worker that.

normally though, it is around 50 to 75 cents per invitation.

That depends on the country you are in and where you are sending to. Check the website of your local postal service for rates.

What country are you mailing it from?

68p.