What is the Parking Assist package and why is it so cheap?

What model are you buying?
It's optional on the LE ($35) and comes standard on the XLE and Limited. If you're getting an XLE or Limited, the dealer shouldn't be charging for it - question it. I've seen dealers try to charge for things that come default - like $135 for a 12.3" infotaiment screen that's standard on a Limited. Question it - it's not a legit charge.

The feature is the front and rear cameras that show your car in relation to the cars & objects around you.

 

It's not the front camera - that's part of a much bigger tech thing, the Advanced Park.

And the rear camera is standard on all trims (and possibly legally mandated by now).

Not 100% sure, as the US and Europe safety bits don't quite align.

It could be the parking assist sensors, although I thought they were standard.

It could also be the souped-up rear camera ("parking assist monitor") that has guide lines, wide-angle setting, camera washer, Rear Camera Detection. The basic (legal minimum?) one is just a camera with no controls.

Or maybe those last two combined?

 

Seems to be ultra sonic sensors that lets the car scream at you when you get too close to an object at low speeds, like when parking. It appears every LE shipped to the US has it. I suspect it was intended to be standard for all, but Toyota was hedging bets in the feature listings in case of parts shortages. The $35 might be a reflection of how likely they thought not having it would be, or it is literally the cost of the single widget that could possibly be in short supply.

At the height the chip shortage, Toyota was offering models without some standard features as an option. This could be a holdover from then in their system.