Can I add a discount to my WhatsApp order confirmation message?

Not without it becoming marketing. A plain order confirmation is a utility message. Add a discount or offer and Meta treats the whole template as marketing.

• Utility templates confirm, update or cancel an order using specific order details.
• They should not promote, recommend, upsell or include offers.
• A template with mixed content, such as an order update with a promo, is categorised as marketing.
• Send offers as their own marketing message to customers who agreed to receive them.