Hi Claudio and congratulations for your plugin.
I have been searching for this feature for woocommerce and I am finding your plugin very useful.
Nevertheless, It have found two problems:
First of all, IMHO, it should calculate and apply the discount over the subtotal of cart before taxes, not after taxes (or, at least, this shoud be left as an option). This is important because, though the amount to be paid by the customer results the same, the calculated taxes are wrong and the invoice is not valid since a legal point of view (I am talking about VAT taxes; I do not know with other kind of taxes). And this is a point if you own a shop.
Second, the discount does not show in the review order page. It appears just while the page is rendering but, finally, it dissapears from the definitive page. Maybe this could be a problem with my theme...
I have reviewed your code and I was able to modify the way the discount is applied, so I am using your plugin after all ;) But I think that this plugin is really a good work and it is a pity that is not usable with VAT taxes. I think it would be worth fixing it.