You've been probably been paying the correct postage Paul. Many bubble envelopes require the surcharge. It depends on the size. If the envelope is any of the following you need the 45cents for less than an ounce.

* Height exceeds 6 1/8 inches, or
* Length exceeds 11 1/2 inches, or
* Thickness exceeds 1/4 inch, or
* Length divided by height is less than 1.3 or more than 2.5.