You can save any order for later by clicking "Save order for later", right above to the "Review order" button.
This will safely store your order. You can come back to your order later by clicking it on your playtest overview page. Draft playtests are indicated by a dotted white outline and are also tagged as "Draft" underneath the playtest name/date.
If you would like to edit the order select the "Continue your order" button on the draft playtest. You can also do this to retrieve the order link and share orders with your team and work together to set up a playtest.