basically, from what i can see and understand, is that you take the original price. lets use the chair.
$139.99 you choose which discount you would like to use either the $20 off or 20% off.
if you choose $20 then
$139.99-$20=119.99
but if you choose 20%
$139.99x0.20=27.99 (meaning you deduct $27.99 from $139.99)
139.99-27.99=$112
now you decide which one you'd choose. i would say 20% off because it costs less.