Please note to check your weights.

[gjs_16]

Generally speaking every time the USPS increases rates there may also exist certain variations in price between the ounces for first class.  For example, before the increase a first class parcel weighing 8 or 9 ounces cost the same, now it does not (now 9 ounces is like 50 cents more).

May or may not make a difference but in my case with 100+ listings in at least a few instances it does.

And then they "revert" back to the old prices in a few months but just for a short while and then back to the new.

The slick 1-2-3 oopsie money grab.

[nobody*s_perfect]

The weight brackets for USPS First Class packages have not changed.  Rates are the same for (1) up to 4 ounces; (2) 4+ to 8 ounces; (3) 8+ to 12 ounces; and (4) 12+ to 15.99 ounces (or to 13 ounces if you're shipping at retail rates).  Packages at 8 ounces and 9 ounces have never had the same postage cost.