Tax hell! If order is not subject to VAT, nopcommerce not "discount" tax.

Ok now it is clear? :)

nopCommerce experts, is a bug or not?

I suppose that it's a bug but it's preferable that an expert confirm us if we are doing something wrong.

Anyway, I have decided to insert prices without tax, this way works fine.

I'm not able to find this information.

Product price shoud be inserted WITH or WITHOUT tax?

You can set your prices on two different ways:

1) Set "Prices include tax:" checbox to true and set prices to products tax INcluded on your store.
2) Set "Prices include tax:" checbox to false and set prices to products tax EXcluded on your store.

The first one is problematic with tax free products.

I am working finally with the second option, all works fine.

This is a bit of a guess but I think the logic is as follows;-

If you are only selling to customers in 1 tax area (yours) then you can select prices include tax. This is in effect what physical shops do.

If you are selling to more than one tax area then you must deselect prices include tax and enter prices ex-VAT.

You/ the store owner should probably read and understand the following document for the UK here

That looks like a whole world of pain. :/

Probably at this point an accountant versed in VAT law would be useful.

HTH