option as discount %

Hello

i need some help determaning how to set up a OPTION as a discount with PERCENTAGE instead of fixed price for a product-option
my products are set up as services not phisical products

i am starting a small local courier service
and i need to have the option if the customer chooses to send the letters all to ONE adress then the discount is 20 % from the total of the price of qty services in quantity

so the example here

1 letter - economic service - is 5 lei (currency)
5 or more is 3 lei / letter so that is 15 lei for 5 letters

now if the customer sends those 5 letters to the same address the discount is 20 %

options

Same adress 20 % discount
1 different adresses 5% discount
5 different adresses no discount

so the main issue here is using the qty discount for the options as percentage instead of fixed prices

i hope there is someone here that can help me out on this issue