Yes, Nettla allows you to set up temporary or long-term promo codes that can be used for a wide range of purposes.
You can make it globally feasible, such as a 20% discount on everything for a specific weekend, or
Very specific with only players in a certain cohort being able to use the promo code for Tuesday morning.
To set up your promo codes, go to Shop -> Discount codes -> Create new and you will find a wide range of settings and rules that you may or may not apply to the code that you are creating.