On each achievement edit screen you will see a new box with the title “Award a discount”. Just checking the option “Award users that earn this achievement with a discount?” you will be able to configure the discount that will be served when the achievement gets completed.
Type of this discount. Available options are:
Fixed Price and
Amount of discount to apply. If you set Discount Type as
Fixed Price this amount will be applied as a flat amount. If you set Discount Type as
Percentage Price this amount will be applied as a percentage. In both cases, you do not need to enter any symbol (currency or %).
Products where discounts will be applied.
Label to show on fees.