As mentioned in OFBIZ-4160 there are different ways to approach tax-calculation on shipping-costs.
While some countries have a fixed price for this, others calculate the delivery just like the delivered good (or a weighted version, if multiple taxrates are applied to the shipping).
As I stated:
[..]it is advisable to introduce a new ProductCategory "TAX_SHIPPING". This could be administered through TAX_AUTHORITY_RATE_PRODUCT for individual tax authorities. Then there could be a condition before my newly implemented logic, to use this tax rate, if it is found, and if it is not, my logic is executed.