Assuming they weigh the same, why should a square envelope cost more to mail than a rectangular one? That’s completely ridiculous. There must be a mistake here.
But its not a mistake. It says it right there in the United States Postal Service Envelope Code:
First-Class envelopes weighing 1 ounce or less require an additional $0.12 nonmachinable surcharge if any one of the following apply:
- It is a square letter
- It contains very rigid items such as wood or metal
- 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
The key adjective here is nonmachinable. In other words, square envelopes, along with envelopes that have decidedly non-paper contents, or envelopes that don’t conform to the USPS golden ratio (see last bullet point), won’t go through the machine.
We assume these troublesome letters have to be sorted by hand, and that costs exactly twelve cents of additional postal worker effort. We also trust that this figure was determined after a lot of careful research.