28 Cent Stamp For 27 Cent Rate

The current additional ounce rate stamp sells for 28 cents while the first few additional ounces on large envelopes cost 27 cents each. USPS logic eludes me. Took me a bit to recall any US stamp de-nominated 27 cents. (hyjphen needed to avoid a blocked phrase).

How about these?

Those were the only 27 centers I could think of, maybe there are others.

The first ounce large envelope rate has been 2x the letter rate for many years, but this summer changed to 2x plus 4 cents, with the first few additional ounces 27 cents instead of 28. Another oddity is it now costs more to mail one 6 ounce letter than two 3 ounce letters.

What do you mean, more to mail one 6 ounce letter than two 3 ounce letters?
There's no such thing as a 6 ounce letter since shape based pricing started in 2007.

Keep adding "large envelope" when typing about letters over 3.5 oz (max for letters) to stay on topic. So, 29c differential on 6-oz large envelope vs. two 3 ounce letters, hardly oddity when you don't forget the premise - a one ounce letter is 73c while a one ounce large envelope is $1.50, now that's a differential more than 29c (more than 100% more in fact).

Regardless of terminology, instead of sending a 6 ounce mamuscrupt in one envelope, I can split it across two smaller envelopes, and the total mailing cost becomes less. I suspect this counterintuitive result occurs because a heavier single envelope requires more manual processing.