You're able to create promo codes under the Discounts tab on your dashboard. The two types of promotions you can create are: a percentage discount or dollar amount discount.
This can be for either a single service/several services or on an entire order.
How discounts affect your profits:
When you create a promotion, the entire discount amount comes from your profits. For example, if you give 10% off of a $149 service, you subtract $14.90 from your profit amount. This is the same for both types of discounts (percentages and dollar amounts).