Don Wildmon and his band of bigots have a handy-dandy web form that you can use to tell Hallmark how awful it is that they’re trying to make a buck by acknowledging the fact that same-sex couples can and do marry. Of course, you can also use that same form to write your own message of support to Hallmark. (Though you may want to use something other than your primary email address, just in case you start getting spam from the AFA.)
Update: As several people have pointed out, there are downsides to using the AFA form besides just spam. A better idea that someone mentioned in comments would be email Hallmark directly. Even better? Buy one of their cards and send it to them!