I am SO excited to have designed my own post-card STDs. First, I found the idea on to use overnightprints.com from a post on weddingbee.com. You can order 100 post cards for $30 (digital print is great), but i got them with a promocode which turned out to only $21 after shipping for 100 post cards (that's 21 cents for a quality STD postcard)! They have templates to chose from but it's much more gratifying if you create your own design. My brother happens to be a photog so he took our pictures (but if i didn't have his help i would've just used various old pics we have) and then I was able to use our wedding colors and play with differnet fonts. Its great quality paper stock and if you want to see it before you order you can request that they send you free samples.
I created my own desgin using Adobe photoshop and went to "Upload Your Design". The site has directions on how to properly upload and instructions for any program to make sure you upload it correctly. The only mistake I made was to order glossy. In retrospect I would've gone without it because it looks much better without (non glossy still have a UV coating so it's not matte).
Also, for the labels, I printed it on clear address labels. For .28 cent post card stamps the ONLY thing available is the one with the polar bear (unless you pay extra at a website like zazzle.com). So, you have to go with that or use the meter stamp. But if you have glossy the meter stamp will smear. I hated the idea of using the polar bear stamp, but in the end i realized it's not that big a deal!
Here's the front and back: