Build and customize Spree's promotions engine — Promotion + PromotionRule + PromotionAction + CouponCode + Adjustment, the bundled rules (FirstOrder/ItemTotal/Product/Taxon/User/OneUsePerUser/Country/CustomerGroup/etc.), bundled actions (CreateAdjustment/CreateItemAdjustments/FreeShipping/CreateLineItems), Calculator classes, coupon batches with CSV export, the v5.1+ advanced rule-based engine, and authoring custom rules/actions/calculators. Use when modeling promotions, building discount UIs, or extending the promotions engine.
