Tariffs are set for classes/types of goods/materials/etc... and Kinder Eggs were banned because they didn't comply to a regulation (small dangerous foreign objects contained in the product - maybe other similar products had to be banned as well but those ones didn't reach the headlines).

In this case the decision applies to a very specific company/product and that decision is not based on a rule/regulation/etc..., I think => on one hand it doesn't have any impact on other similar companies/products, on the other hand it scares me that a single person is able to enforce on-the-fly these kind of decisions.

Obviously, an ad volarem tax (https://en.wikipedia.org/wiki/Ad_valorem_tax) that is a tax that is proportional to the market value of a transaction like VAT wouldn't do anything.

But there are other kinds of taxes.

Eg fuel taxes in many countries are usually per litre, not per dollar of petrol. See eg https://en.wikipedia.org/wiki/Fuel_tax#Tax_rates

Carbon taxes proposals are also usually imagined to apply per kg of carbon emitted. Exactly because there's no prior price of carbon emissions.