I do my own custom clearances.
In the Postnord case it probably stems from the fact that mainland China, in the fees and tariffs scheme of the UPU, is a third world nation, and as such is exempt from paying a lot of the transfer costs for international mail. Sweden and Denmark, OTOH, are in the highest charging level, and then have to carry all the costs of that operation. Postnord basically get a shitload of $1 parcels with fake IC's, banana jacks, sunglasses and dildos dumped on them, and are expected, according to UPU rules, to deliver them for next to nothing.
Shipments that have declared value under ~ 75€ are exempt from customs fees, but not VAT free. And, the rules allow the import handler to charge a fee. The VAT goes to the tax authority, the handling fee to the shipper. And no, they can refuse you the right to handle customs processing.
The loophole of course is the "Czech warehouse" which is a paper-thin store front to the same Shenzen merchant, but inside EU. Only one VAT process, and no customs. Most importantly, no handling fee.