To delete a draft, tap into your Post Box and into your drafts. Swipe to the left to expose the Trash button.
To delete a draft on the iPad, first make sure that you're in your Post Box. Tap the 3 lines menu in the top right to expose the menu and tap the Post Box from there.
In Drafts, switch over to List View by tapping the button in the upper right hand corner. Then, you can swipe left on your draft to view the Trash button.
** Please note you can only delete drafts and not sent cards, and you're only able to do this on the computer.**