First Class Mail rates are not just determined by weight, but by size too.  Especially if over one ounce.

For example, an envelope weighing under one-ounce, AND under 1/4 inch in thickness AND/OR has a rigid object in the envelope, is subject to a 'non machinable' surcharge of $0.17 on top of the First Class Mailing rate. This surcharge can apply if any of those characteristics below are also me.

If the envelope exceeds 3.5 ounces, you would be subject to First Class Parcel rates, which are shape-based rates, and subject to much higher mailing rates.

Other envelope characteristics that make your mailing subject to the non-machinable surcharge:

• It is a square letter
• It is too rigid – does not bend easily
• It has clasps, string, buttons, or similar closure devices
• It has an address parallel to the shorter dimension of the letter
• It contains items such as pens that cause the surface to be uneven.
  
• The length divided by height is less than 1.3 or more than 2.5