You can change your postcard only until you press 'Send'. But the moment you press 'Send', your postcard is gone and you'll no longer be able to change it. You can reuse the same card and send it again, to the same person or someone else. You can do this up to two weeks after you send the postcard the first time. You're able to access a copy of the card and edit it in the app. You'll have to pay for this subsequent sending.