I'm pretty sure that you are correct. I made a chart for mailing, because it changes just about every year, and I have to keep adjusting, what's what and how much. Also I weigh each one and probably never mailed a chip that wasn't at least 2oz.

So a Non-Machinable Butterfly would cover a 1 oz envelope. Then additional first class, whatever that is today 🤔 for the next oz. including at 3.6oz to 5.0 oz things change again. To rigid envelope rates.

But aside from all of that and Pirate Ship and the USPS often not knowing their own regulations, I've never had an issue yet, with a Butterfly and a first class forever, or a slew of old commemorative, adding up to the right rate. And I mean, no Butterfly, just stamps that are the right Non-Machinable rate, and a clearly marked mailer,. I printed labels and I add them, from and back, just in case.

ps If you put the address across the short side, then the envelop, theoretically becomes non-machinable. And if you do that with a postcard, it becomes First Class.

Officially:

Letters that meet one or more of the nonmachinable characteristics in DMM 101.1.2 are subject to the $0.49 nonmachinable surcharge. https://pe.usps.com/cpim/ftp/manuals/dmm300/101.pdf

Domestic Mail Manual • Updated 11-3-25
101101.1.2
Retail Mail: Physical Standards for Letters, Cards, Flats, and Parcels
101 Physical Standards
1.0 Physical Standards for Letters
1.1 Dimensional Standards for Letters
Letter-size mail is the following:
a. Not less than 5 inches long, 3-1/2 inches high, and 0.007-inch thick. For
pieces more than 6 inches long or 4-1/4 inches high, the minimum thickness
is 0.009. (Pieces not meeting the 0.009 thickness are subject to a
nonmachinable surcharge under 1.2f.)
b. Not more than 11-1/2 inches long, or more than 6-1/8 inches high,

or more
than 1/4-inch thick.

c. Not more than 3.5 ounces. (Charge flat-size prices for First-Class Mail
letter-size pieces over 3.5 ounces.)
d. Rectangular, with four square corners and parallel opposite sides.
Letter-size, card-type mailpieces made of cardstock may have finished
corners that do not exceed a radius of 0.125 inch (1/8 inch). See
Exhibit 201.1.1.1.
1.2 Nonmachinable Criteria
A letter-size piece is nonmachinable if it has one or more of the following
characteristics (see 601.1.1.2 to determine the length, height, top, and bottom of
a mailpiece):
a. Has an aspect ratio (length divided by height) of less than 1.3 or more than 2.5.
b. Is polybagged, polywrapped, enclosed in any plastic material, or has an
exterior surface made of a material that is not paper. Windows in envelopes
made of paper do not make mailpieces nonmachinable. Attachments
allowable under applicable eligibility standards do not make mailpieces
nonmachinable.
c. Has clasps, strings, buttons, or similar closure devices.

Or from other information: (which makes the parcel interpretation make more sense)

Large envelope-sized pieces that are rigid, nonrectangular, or not uniformly thick pay parcel prices.

And as I've discovered:

For weights over 3.5 ounces, see Large Envelopes prices.

So the whole Butterfly and Non-Machinable is pretty limited in actual usefulness. But I like it, I have the forever stamps, I have the 1950s stamps, and I think sometimes it baffles the people who are working in the sort centers, and they are too busy to bother with all the time and regulations, they just pass it through. Which means, I just drop them in a box. I'm tired of people behind the counter, telling me, I don't have them right.