Interesting idea, but tax is inline?

I was thinking about account.invoice. You can add tax there. But if you need to do it from Sale Order/Purchase Order, I think you need to build a new functionality.

Just to clarify, in Account Invoice you can add tax at the invoice, not invoice line.

I added a new tax position in the account.invoice (page Other Info?) but this tax is not included in total invoice value (to paid)? This way or other?

PS. V7 or V8? my system V8.

I've used this method in v7. I think it should also work in v8 because they haven't change the account.invoice model much. Have you click on the (update) link next to the Tax before the Total?