"That's the polarity adapter plug. Most things are center-pin negative. There are some things out there (guitar pedals and various other things) that use center-pin-positive"
This is plain wrong. It's the other way. Seeing tech support getting it that wrong makes me worried.
Most equipment uses center pin positive, shell is negative.
There is one major exception and some minor exceptions. Musical effects that were powered by 9V batteries or mains adapters from Japan standardised on the japanese style "center negative", which is the opposite of the common standard.
In 2024 the only remaining products with this are guitar pedals. They need 9V. No other power supply input can be found now that is center negative. Back when an ipower was 39 or 49 USD, it was still a valid proposition for guitar players, especially it it would power multiple pedals without causing noise.
Historically some japanese portable CD-Players or K7 Players may also use this, as may some older Keyboards and/or Electronic Piano's or portable karaoke gear, all Made in Japan and most likely at a point where they should be considered museum pieces.
Plugging in the polarity the wrong way may damage the device it is plugged in. Don't do it.
I have no other comment as to what is inside the packages of a given ifi power product.
Back when I had something to say, I suggested USB Adapters to be included with 5V, the "Guitar Pedal Adaptor" with 9V (and ideally a breakout cable to power multiple pedals), nothing extra with 12V and only a 2.1mm Pin Adaptor, as this is mainstream.
There are so many alternate options, leave it to e-bay. No matter how much you put in, there will always be an irate customer that needs the one single adaptor not included and seeing all the others included get's upset that he is personally sleighed.
Thor