Has anyone run into this?

It seems like suddenly the USPS is embargoing or slowing shipments of batteries for some reason.

I ordered a new battery from iFixit and they've always shipped quickly in the past but this time the battery I ordered for my iPhone 6 took over 2 weeks to arrive. USPS tracking said I was supposed to have it in 3 days but after a week I contacted the USPS and iFixit and USPS had some supervisor trying to track it and said basically it was "stuck" somewhere due to it being a lithium battery. iFixit shipped me another one via UPS which came in like 2 days. The original order coming via USPS FINALLY arrived almost 2 and a half weeks later.

I bought some button batteries for my wife's Mazda keyfob from eBay and it's looking like the same exact thing is happening again. Tracking said it would be 3-4 days, they are being shipped via USPS and it's been a week and a half now and no batteries yet.

WTF is going on here?

No idea.
I get all types of batteries shipped in all the time from a number of sources and shipping methods with no problems.
But most come UPS or Fed Ex and are usually UPS-Backup Batteries.
Maybe it looked "suspicious".

I would assume it has something to do with having too many "hazardous" items shipping at one time.

I would say they get even more regulated if they have the shipments going through a contract with a passenger airline.

They always ask me at the USPS counter if my package contains anything liquid, fragile, perishable or hazardous, like batteries or perfume. From that, I have come to suspect that they no longer accept ceramic, battery powered perfume sprayers for shipment.

Lithium batteries are "dangerous" items at the USPS. With some reason, as cheap high capacity cells can leak gas (or more properly, NOT vent gas properly). Some can overheat dangerously and are a fire hazard.

That is what causes those cheap Chinese hoverboards to catch fire - shoddy lithium batteries.

USPS uses passenger aircraft (as well as UPS planes) for their expedited mail. Lithium and lithium ion batteries are required to ship via ground because of this. USPS ground is brutally slow. There is a lot of confusion among clerks because the labeling is complex and there is a difference in the handling/labeling and actual volatility of lithium vs li-ion batteries. At least that's how it was explained to me at the carrier academy...