I added the {post_id} merge tag to access the id of the new post that’s created.
I use the advanced post creation add-on to create new posts when users submit the form. However, the merge tag remains blank in the user’s email/notification.
As for the merge tag, where are you using it? APC feed are processed via separate background request, so the post ID won’t be available if you are using the merge tag in the confirmation message, or a notification sent on the standard form submission event.