Click on your name in the top right corner of the Hallmark eCards site, select Account and find the Subscriptions tab. Click on the Cancel Auto Bill button. This would mean that your subscription will lapse on the subscription expiration date.
NOTE: If your account was created through iTunes, all billing and account issues must be handled through the iTunes app. Step-by-step instructions on how to manage your iTunes account can be found by visiting: http://support.apple.com/en-us/HT202039
If you have further questions, please visit https://www.apple.com/support/itunes/account/ for more information.