Unfortunately at this time we don't have an option for you to add an attachment to your card. If you have an image or document that your need your guests to print or fill out, you can save your attachment as a photo and add it to your mailing as an insert.
On your Customize page, open the "Other Options" menu that appears to the left of your card image and choose "+Photo Card." You can then upload your attachment as an image (be sure to save it as a .jpg or .png).
Your recipients will be able to right-click on the image to save it when they view your card in their browsers.