Going by what I see on the label, that bubble mailer weighed 2 ounces and is more than ¼" but less than ¾". Without Delivery Confirmation, it is considered a large envelope (flat) and postage would be 97¢.

With Delivery Confirmation your 2 ounce bubble mailer is considered a Package. Postage would be $1.30 + 75¢ for First Class Delivery Confirmation for a total of $2.05.

Your package clearly says "USPS First Class Mail" so the clerk at your post office was wrong to charge you Priority!