The purpose for de minimis exceptions to tariffs is that below some level of value it is more trouble and expense than it is worth to collect import duty on low-value parcels.
Of course, the appropriate level for this is debatable as a political and practical issue.
With automation and prepaid duties the practical level is almost non existent. The spot checking workload does not change and the computers work almost for free.
There will be several extra steps involved with the atoms (i.e. not electrons not money). Off the top of my head there will be...
Locating a package and matching it to electronic records
Sorting packages that do/don't match; even simple inter-bank
electronic money transfers are surprisingly difficult to reconcile; there is a whole sub-industry devoted to it.
Raising extra charge invoices with the destination.
Customer service enquiries about them
Storing packages awaiting payment of extra fees
Locating the specific package when the fees have been paid.
Printing invoices for the extra fees (happens already)
Returning "unwanted" packages to the source, or destroying them
Debt collection for the reverse transport fees.
And I'm sure there are more

Now DDP ought to avoid many of those, but people are indicating the seller won't pay the duties.
EDIT: as expected I was too optimistic - JPortici has already run foul of another. He's been hit by US customs declaring the clearance code was wrong. That leads to the "CSR", "unwanted package" and "reverse transport" problems predicted above.
https://www.eevblog.com/forum/chat/tariffs/msg6037101/#msg6037101