promote_spn's main campaign was actually to send a few hundred postcards to the CW at once, all encouraging SPN's renewal. That campaign ended before their support campaign. While writing a short postcard is the best way to get a network's attention (see here), and I encourage you to write them to encourage its renewal, the network has been behaving positively towards SPN. If you want to send some postcards of support, I would suggest writing to them with confidence in the show and leaving "save" out of the message.