"if the system actually works properly it should be impossible to charge excessive postage making a low DSR impossible to do"

I agree, but how well does the system work?

Buyers can leave a low score for a variety of reasons, which can be honest ones. Such as being asked to pay more postage after the listing has ended, in order to receive the item at all; a misunderstanding as to what a "reasonable" handling fee might be; a disagreement on excessive postage...450$ is well in excess of most postage costs, so I can see the system flagging that.

I would be interested to know when it is flagged as too high. 50$? 200$?

How the system determines "excessive" would also matter (as in weight, or size).

So depending on the threshold, it would be possible for sellers to have excessive postage in a listing.

I have seen 25$ post for a few pennies, for example.

Some reasons are unfair to the seller, I agree, but not all would necessarily be false, as the system does seem to have flaws.