Here it is from the post office's website - the 3/4" rule, which applies to delivery confirmation. The problem we were discussing in the other thread had nothing to do with the weight of padded envelopes - it had to do with the added service of delivery confirmation!:

Delivery Confirmation may be used with the following services:

First-Class Mail® Parcels
Priority Mail®
Package Services Parcels - Includes Parcel Post® , Media Mail® , Bound Printed Matter, and Library Mail.
Standard Mail Parcels - must be less than 16 ounces and mailed at the electronic rate.
Parcel Select® - must be mailed at the electronic rate.

Note: All items (except Priority Mail) mailed with Delivery Confirmation must measure at least 3/4" at the thickest point, or be in a rigid box.

I never encountered this problem because I don't generally use PayPal labels on small envelopes. But the problem, I think, is with PayPal's calculator - it automatically adds delivery confirmation, but never asks how thick the envelope is. As a result, it charges the sender the weight-based rate based on the first-class parcel rate ($1.13 for first ounce + 17 cents per additional ounce + 18 cents for delivery confirmation), and our customers may end up with a postage due notice because it should have been based on the priority mail rate ($4.60)! This stinks, and you can bet post offices across the country will start to see this as a great way to enhance their revenues.

So, unless you have an envelope exceeding 3/4 inches at some point, or a rigid box, avoid purchasing delivery confirmation unless you are prepared to pass along priority mail rates to your customer!