Hi Diego,
1. No price on offer is before tax.
2,3,4. Not sure if its different in Italy, but in the US I am able to define tax rates in my PayPal account so that if a user has a North Carolina billing address, taxes are applied to the order at PayPal. Same with google. I don't think it would be the right plan (at least currently) to pass tax data to PayPal because we send the user there without knowing his billing address, so we do not know ahead of time whether tax is applicable. We get the customer address after the order is processed (or at least authorized) for both PayPal and google, not before. hopefully its possible for you to configure your PayPal account to apply VAT for orders in Italy. Currently the internal mojoportal tax rate is (I think) only applied if using Authorize.NET or PlugNPay because in those cases the order is processed without the user leaving the site and we do collect his billing address in order to determine if the order is taxable. I do not know if it currently works based on Country alone as I have not tried it and not sure if its coded to do that. I do think it is possible to pass the tax to PayPal and maybe google too but that would be implemented when I implement support for physical shipment. PayPal does have a way to rdirect the user back to your site after authorization and you can calculate shipping based on the shipping address and adjust the order accordingly before completing it, and I think it is also possible to add tax yourself, but its easier to let PayPal do it. So as I said, I'm planning to work on that as part of the implementation for supporting shipped products but have not yet completed that. Ultimately I'm sure there are improvements needed for the internal tax calculation system and also we should make it possible to integrate 3rd party services for tax calculations as well.
Hope it helps,
Joe