Did you ever finish the extension?
Here in Finland the shipping fee’s VAT should be calculated by the ratio of the products bought.
So let’s say Product A has VAT 23% and Product B has VAT 9%
Customer makes the following purchase:
Product A 40 eur, incl VAT 23%
Product B 60 eur, incl VAT 9%
Shipping fee 10 eur incl VAT
Shipping fee’s VAT should be calculated like this:
4 eur of the shipping fee with VAT 23% 0,75 eur
6 eur of the shipping fee with VAT 9% 0,50 eur
Shipping fee’s total VAT 0,75 + 0,50 = 1,25 eur
I don’t know if this is how it goes in other EU countries, but this should apply here in Finland. At least this is how I understand this thing.