It has been answered before.
Yes the extra 20% is over and above any other discounts. However your maths is incorrect 35% combined with 20% does not give 55%. The calculation is that you multiply the discounted rates, not add the discounts, (1.0-0.35)*(1.0-0.2) => 0.52 of the nominal price, i.e. a 48% discount.
And it is not off all cards - only those for the latest 10 days.