Omni-Channel Promotion Engine

Design a system for an e-commerce platform to calculate order discounts. Rules can be complex: 'Buy 2 get 1 free on shoes', '10% off for first-time users', and 'Free shipping for orders over $100'. Some discounts are stackable, while others are mutually exclusive (e.g., if the user uses a coupon, they don't get the automatic 10% discount). The engine must allow the marketing team to frequently add new promotion types without code deployment for every rule.