You can create the special promotions to waive the domain registration fee in the Billing Director - Discount Manager - Promotions. Such promotions allow setting up the minimal subscription period to apply the promotion, the period of time to waive the domain-registration fee if the promotion was applied, and maximal amount to be waived.
In addition, you can create the coded promotions that can be activated by a coupon code entered by a customer in the HSPstore.