There are two ways of awarding commission, fixed and percentage.
Fixed Commission guarantees the affiliate a fixed amount, usually used in cases where the goal itself has no monetary value such as a 'Sign up'. You may also consider Fixed Commission if the value of a sales is large or where your profit in selling an item is also fixed.
Percentage Commission takes the value of a sale and calculates a percentage of that value as the commission to pay the affiliate. This is perfect for e-commerce websites. You should ensure that your profit in selling an item allows for this type of commission determination.