The USPS Butterfly stamp is designed to cover the 1-ounce First-Class Mail postage and the non-machinable surcharge for items like square greeting cards or other non-standard shapes. The non-machinable surcharge is added to items that cannot [or should not] be processed by automated machinery due to their shape or rigidity. As of April 2024, the Butterfly stamp costs $1.19 and covers both the basic postage and the non-machinable surcharge.

If the mail piece is 2 ounces, a 2nd ounce stamp is needed, but not a second surcharge.

The purpose of the butterfly image is to allow the automated machinery (yep, a non-sequitur) recognize that the mail piece in NON MACHINEABLE. I do not know if the required printing of "NON MACHINEABLE: on the cover, lower left corner, is also readable by the automated machinery (yep, still a non-sequitur), but, when I mail over-the-counter, the mail clerk will hand-cancel the mail piece and throw it into the same box that all other letters are tossed into. Again, another non-sequitur.

Envelope thickness is very important! NON MACHINEABLE does not let you mail an envelope thicker than the template a mail clerk will use to check for thickness.

Securing a chip into an envelope is also very critical. Do not allow a chip to slide around inside an envelope. There are a variety of methods to secure a chip so that it cannot slide around (and out the edge of the envelope) but DO NOT use tape directly on the chip!!!

Lastly, I have used standard postage stamps that total $1.19 for a 1 ounce, NON MACHINEABLE mail piece, meaning a butterfly stamp is not required to cover the 1st ounce and surcharge.

Jim

ps. But then again, the Post Office is what it is!