By far my favorite way to add postage to your invitation envelopes is to use vintage, unused stamps. They come in every color and in every denomination from one cent and up. You can mix and match different styles and colors to come up with an effect that works with your color palette. One of my favorite vintage stamp vendors is Virginia Sauder at Verde Studio. You can choose from a variety of themes like history, nature, love, botanicals etc. She can also put packages together for you if you tell her your needs.
Your next option is to purchase new stamps from the post office. Their "love" themed stamps are not my favorite. To be honest, I don't find them imaginative and vintage enough to my liking. But I do love many of their historical and botanical themed stamps. If you are having a very vintage wedding, you might use a mix of their WWI and Classics Forever series.The last option is to have your own custom stamp designed. You can use your couple's logo, an image from your invitations... anything. The main drawback about this is that a standard .55 stamp can cost up to $1.30 each. More importantly, they come with a very unsightly black and white QR code on the design. You can purchase these from Shutterfly, Minted, Zazzle etc. I say stay clear from these-- you can spend the extra expense elsewhere and the black and white code is really a drawback.
Once you drop your lovely little packages off at the post office, they are going to be machine sorted and stamped. To ensure your invitations do not bend or smudge, you can ask your postal worker to hand cancel each one. This ensures that they will not rip or tear in the stamping and sorting process. If you are using wax seals, this is a must. Hand canceling can only be done once you have sealed, stamped and addressed your envelopes. Hand canceling does take a bit of time and some post offices may charge for this. Make sure you ask which day is the least busy at the post office.