Delivery confirmation is available for packages more than 3/4 inches thick OR Priority Mail, any thickness.

If you use a PayPal label (which automatically includes delivery confirmation) for a first class mail package, you have to make sure the package is at least 3/4 inches thick.

I have known of people who received packages less than 3/4 inch thick who were charged "postage due" at the expensive priority mail rate. They were some of my customers, and I felt bad and reimbursed them. I am now more vigilant to make sure my padded envelopes include cardboard or other filler to reach the minimum thickness.

I believe they impose the thickness requirement so carriers don't have to look at every thin piece of mail to determine whether they need to scan it.