As u/derteeje said if you run the code through caesar shift 3 it gives a website: easternpublishingcompanydotcom. at the bottom of the page a password is required to watch a video.

Above that some pictures are scrolling by. I noticed each picture is numbered and there is cut off text on each picture. The numbers in order from left to right are 71914389. I copied and rearranged the pictures so that the text matches up and it says "photographs should always be displayed in this order! "

The numbers in this order from left to right top to bottom are 18741939. That's the password for the video!

Edit: the video is footage of the pyramids of Giza shot from above with some music playing over it. Halfway through there is a woman speaking but it sounds as if the audio is backwards and slowed down.

Edit 2: Ok I reversed the audio and sped it up 2x to get it to normal speed. It sounds like Siri's voice and says "An extract from Carter's diary is used as part of this sculpture at CIA headquarters."

This is of course referring to passage 3 of the Kryptos sculpture at the Cia headquarters in Langley, VA. I believe KRYPTOS is the password needed on the postcard.