I would note that a postcard is not secure enough for sending something like this. While it is unlikely that someone in the USPS will take a picture of the postcard, then type in the key when they get home, there's basically no way to stop that from happening, or fixing it if it does happen. As long as it's in an envelope, it should be fine, though.

Basically, take the existing setup, connect one extra itemduct to the itemduct connected to the harvester, set the extra itemduct to dense mode, and connect it to a chest. Anything that can go to the planter will go there, and anything that can't will end up in the chest.