yeah, you can't have delivery confirmation unless it meets the requirements of a first class package. It's not available for first class envelope rates. If your label has delivery confirmation and it's not package thickness, they charge the much higher Priority Mail rate. They charged postage due to one of my customers, and I felt terrible about it.

You may be able to cancel the postage if you purchased it on-line, or ask at the post office if you can re-send with the added padding. In the future, as you have noted, just add bubble wrap or cardboard to hit the magic 3/4 inch thickness.