How much postage (how many stamps) will you need?This question comes up a lot. I've already written about mailing save the date magnets out, and will focus just on postage here.
The main question when figuring out postage is: how much does it weigh? Postage cost is based mostly on the weight, so that is the logical place to start.
There are some quirks that you might want to be aware of, though.
The one that often surprises people, even though it has been a rule for quite some time: a square invitation such as 5.5x5.5 inches, will cost about 20 cents more to mail out than a rectangular invitation, such as the common 5x7 inches.
Most save the date cards will not require more than a typical forever stamp (first class) when you are mailing with the US postal service, because they are usually fairly small (5x3.5 inches, 5.5x4.25 inches) and will weigh under an ounce.
There are some obvious exceptions. For example, if you are mailing something quirky like a message in a bottle save the date, and therefore need to mail it out in a circular tube, that is something that you need to take to the post office and get a postage price quote for.
If your save the date magnet is one of the larger options (such as a boarding pass), it probably will be heavy enough to require more than a first class stamp. The next most common stamp used is... 68 cent stamp, for those letters that are over an ounce in weight, but under two ounces.
Many simple wedding invitation sets will be under an ounce (simple, as in, a flat 5x7 inch invitation + a flat 5x3.5 inch response card) and will only need that first class stamp. On the other hand, many invitations are just over an ounce. If you went with a layered invitation, a folded invitation, or have a lot of extra information cards, there is a good chance that you need the 68 cent two-ounce stamp stamp.
Some invitations will require even more postage. The popular pocketfold invitations come to mind. Even if your pocketfold invitation set weighs in at under 2 ounces, you will probably be hit with the USPS 21 cent "stiffness" or "non machinable" charge. As of 2016, there is really only one 89 cent stamp available through the USPS store, a "Henry James". Yes. It's true. If you just don't want the Henry James, you could try a 68 cent stamp + 21 cent stamp, or, two regular forever stamps.
It is actually also possible to get custom postage made printed with your own photographs or artwork, but this does naturally cost a little more than ordering available stamps through the post office.
All in all, weight matters the most when it comes to postage, but if your invitation has a lot of layers, a bumpy bow, or is square shaped, expect another 21 cents over what postage would be if you just went by the weight.
PS: With wedding invitations, don't forget that you need another first class stamp for the RSVP card envelopes!