Yes! You can always save what you have started and we'll keep it in your account until you're ready to send. You will need an account in order to save your invitation or card.
If you are designing an invitation, the first step would be to click the "Save and continue" button to save the design (don't worry, you'll be able to change it before sending).
If you are not signed in, click the "Save for later" button on the following page.