I found this idea on Pintrest a while back, I thought it was cool, but didn't honestly think it would work. This is a post card I sent to my sister via USPS. It was made using a big sponge cut into a wedge shape. I created a narrow channel using a utility knife. The whole thing was then spray painted brown. Walking into an art supply store and telling them you're looking for chocolate cake brown is interesting. After that dried I traced, cut out and attached the postcard component. I was proud to have thought of writing on it before gluing it on. Once the glue dried it was time to frost the cake. The frosting is made using caulk. I actually bought a caulk gun to frost a sponge. Weird. Using a ziploc for piping, I filled the channel and smoothed it, then got to work on the rest of the cake. First you cover it, then you swirl it using a toothpick or in my case a coffee stirrer. I didn't really think the end result would be accepted by the post office. After a raised eyebrow postage was attached and 5 days later it arrived in my sister's mailbox!
This is the tutorial I followed: http://www.sheknows.com/living/articles/965787/diy-cake-postcard-tutorial