No tariffs were applied to your two Chinese kits from outside of China because of de minimis. Since no tariff charge is "planned" for a de minimis item, the country of origin is not required to be declared. You are apparently misunderstanding the language regarding formal clearances. All international mail entering the US goes through customs. USPS hands it over to CBP (Customs Border Protection) who inspect it, then hand it back to USPS for delivery. De minimis items are handled differently by CBP than larger shipments. CBP is concerned with whether it is a prohibited item (food, drugs, firearms, etc.) not with where it originated from. If it is not prohibited, it clears. Starting August 29th, there will be no de minimis for any country, and you will notice in the link Ed originally posted that the country of origin will now have to be declared. Steve is correct; items will then be tariffed according to where they originated from.

Again, if you read the link Ed originally posted, for the next six months the courier may choose to pay by either Ad Valorem or by flate rate, and it is the courier who chooses, not the sender nor the receiver, since they are now required to remit the payments. After that six months, all couriers must use Ad Valorem, and again it is not up to the seller or receiver. With the country of origin now on the paperwork, tariffs will be assessed based on that, not where the item is mailed from. Since all international mail does go through customs, and they can (and routinely do) open items up for inspection, where it was manufactured will be definitely determined if CBP desires. USPS indeed don't have that ability, but it is CBP doing customs, not USPS.

What has changed that you should understand is that there are two ways to ship items into this country...Delivery Duty Paid (DDP) or Delivery Duty Unpaid (DDU.) And that was up to the person who shipped the item. With de minimis, things were mailed DDU since there was no duty to pay. Now, without de minimis, a duty must be paid, and the new regulations force all incoming mail (this is only for mail, if you read up on it) to be sent DDP, meaning the seller must collect the duty from the buyer and hand it to their courier who pays the duty to CBP upon the shipment arriving in this country at customs. For internationally mailed items, DDP is now mandatory, and DDP shipments are a specific category within CBP, with their own specific rules.

It is complicated, thus confusing, therefore understandingly easily garbled. After it gets put into effect, many things will become clearer.