Shape, weight, and embellishments determine the cost of your wedding invitation postage. The standard rate for mailing a letter is $0.55. This is true for your typical rectangular lightweight letters. Therefore, most save the dates will only require this much postage unless you include additional embellishments.
However, most wedding invitations weigh more than 1 ounce. Each additional ounce costs a $0.15 on top of the standard rate. Are you using a square envelope? Add another $0.15. Additionally, there are some embellishments that add to the cost. If you have a wax seal, you can expect a higher cost. This is also true for some ribbons tied a certain way. These add to the cost because they cause a variation in the thickness of the envelope. Consequently, they are non-machinable.
By now, I’m sure you’ve figured out what that means…add $0.15!
While this gives a pretty good idea of what your wedding invitation postage will cost, I always recommend getting a full suite weighed at the post office. Getting it weighed at multiple post offices is even better!