These charges have been introduced, primarily, to make it more expensive for EU citizens to buy from Chinese websites that dump dangerous stuff on the European market.
As usual, the European Commission did not have the guts to introduce these charges for purchases from these sites. Consequently, you are charged for purchases under EUR 150, irrespective of their non-EU origin and content. The expectation is that the more reliable sellers from countries that are not dumping will charge the fee and provide appropriate documentation.
Reality, likely, is that when you buy EUR 10 worth of stamps, you get charged EUR 3 per line item plus a EUR 12 administrative fee. Or, in the case of
ebay, they will provide an all-in service for EUR 25. And with a bit of luck, you pay it on every item.
Over EUR 150, charges already applied. And for many items duties were levied on every value. Every now and then, a brain-dead dummy at Tallents House does not correctly fill out the IOSS form and you pay VAT on new issue stamps through Tallents House and you, still, get charged 9% plus a minimum of EUR 12 for clearing.