Zo already gave you the answer, but just reminding that the dynamic discount also applies to activation fees.
And since this is the scrooge forum: By my calculations to get the lowest activation fee per plugin you either need to activate two plugins and use a $25 voucher ( (2 x $20 x 0.8 - $25) / 2 = $3.50 per plugin) or activate 10 plugins and use a $50 voucher ( (10 x $20 x 0.4 - $50) / 10 = $3 per plugin). 6 plugins and a $25 voucher is another sweet spot with $3.83 per plugin, but with every other combination you need to pay over $4 per plugin.