What makes any means of communicating with politicians the most effective is to make it part of an organized movement. Politicians can safely ignore individuals. But, ignoring an organized movement could cost a politician the next election. So, when you identify yourself as being part of an organized movement, you add more power to your message. That is why telling politicians that "GrassRoots GunRights speaks for me!" is so important. It lets the politicians know you are part of an organized movement, and that when it comes to amending a bill, you want the politicians to do as GrassRoots GunRights tells them it should be done.
There is something special about using post cards when trying to influence politicians. There is much that a post card can do that an email can not do.
Post cards take up space on a desk, and in a waste basket. Post cards require staff to handle them, and a mail room to sort them. Politicians will see piles of post cards on the desks of other politicians. Thus, politicians can not easily ignore the post cards from hundreds and thousands of voters. Post cards are an "in your face" means of communicating with politicians.
But, emails can be easily deleted with the click of the mouse. Emails do not take up space on a desk. Emails do not have to be delivered through the mail room. Other politicians can not see the emails sitting in another politician's computer. Thus, emails are much easier to ignore because emails are not an "in your face" means of communicating with politicians.
Unfortunately, we do not always have time to send post cards. Sometimes we need to use email because it is much faster. And sometimes, we use both so as to give politicians the old "one, two" punch.