Forget Postage Rate increases, looks like USPS is back to randomly pulling packages in certain locations, and just slapping rate adjustments on them, whether they deserve it or not.
Several years ago, I noticed specific regions were doing this, more then others, right about the same time.
Opened my Stamps.com account and noticed my account was in the negative. That's a red flag for an adjustment being made. Racked my brain. Did I send something out that got hit with a bubble adjustment? Was the package too long with the new specs.
No. Was a 4 ounce package holding a $6.95 card.
https://www.ebay.com/itm/234406729655
Shipped to Lilburn, Georgia, and delivered last week:
USPS tracking# 9400111108033861214586
They got me for an additional $15.72. 
WTF!!
Claimed I used incorrect packaging, and decided to finish it's journey in a Large Flat Rate Box. Didn't even bother changing the weight of the package.
I pack my cards solid, in small 6x8 envelopes with rigid cardboard reinforcement inside. Generally bomb proof, but light.
Ended up costing me almost 20 bucks to ship that card to Georgia.
I filed an appeal, but last time I did that I never even got the courtesy of a response from them, so not holding my breath.
So, check your accounts for random rate adjustments. I don't get them often, but when they do, they usually start to come in bunches.