DHL mysterious taxes. Staff cannot explain

I bought an item online which attracted 20% import duties. I was aware of that and happy to pay that. When My invoice arrived, DHL has charged me double that and the rest of the costs are hidden in a figure called "tax amount and advance payment". When I call to query this, their staff doesn't know what it relates. They keep guessing its VAT but the calculations do not match this assumption. Then they say its some other tax, I ask what tax this is and how do I calculate it, they still cannot tell me. ***** business practices. Charging clients "tax" that the Revenue knows nothing about.