Bug Report

Description

The buy-get (buyget) promotion engine ignores the application_method.type field and always calculates the discount as a percentage, even when the type is set to "fixed". This causes wildly incorrect discount amounts when using fixed amount discounts with buyget promotions.

Expected Behavior

When a buyget promotion has application_method.type: "fixed" and value: 50000 (e.g., 50,000 VND), the target item should receive a fixed discount of 50,000 VND per unit.

This is consistent with:

• The official documentation example which shows buyget with type: "fixed"
• The ApplicationMethod data model which states: "If it's fixed, the promotion discounts a fixed amount"
• How standard promotions handle type via getPromotionValue() in @medusajs/utils/dist/totals/promotion/index.js

Actual Behavior

The engine always calculates: amount = applicableAmount × value / 100

So a value: 50000 with type: "fixed" is treated as 50,000% discount instead of a fixed 50,000 VND — resulting in an astronomically large discount.

Root Cause

In packages/modules/promotion/src/utils/compute-actions/buy-get.ts, the applyPromotionToTargetItems function hardcodes percentage calculation:

const applicablePercentage = promotion.application_method?.value ?? 100
const amount = MathBN.mult(applicableAmount, applicablePercentage).div(100)

It never reads promotion.application_method?.type to determine whether to apply a fixed or percentage calculation.

Compare with standard promotions in @medusajs/utils/src/totals/promotion/index.ts:

function getPromotionValue(promotion, lineItemAmount, lineItemsAmount, lineItem) {
  if (promotion.type === ApplicationMethodType.PERCENTAGE) {
    return getPromotionValueForPercentage(promotion, lineItemAmount)
  }
  return getPromotionValueForFixed(promotion, lineItemAmount, lineItemsAmount, lineItem)
}

Standard promotions correctly branch on type. Buyget does not.

Steps to Reproduce

1. Create a buyget promotion with type: "fixed" and a numeric value (e.g., 50000):

await promotionModuleService.createPromotions({
  code: "BUY1GET1-50K-OFF",
  type: "buyget",
  status: "active",
  application_method: {
    type: "fixed",
    target_type: "items",
    value: 50000,
    buy_rules_min_quantity: 1,
    apply_to_quantity: 1,
    max_quantity: 1,
    buy_rules: [{ attribute: "products.id", operator: "in", values: ["prod_buy"] }],
    target_rules: [{ attribute: "products.id", operator: "in", values: ["prod_target"] }],
  },
})

2. Add qualifying buy and target items to a cart
3. Observe the computed discount amount — it will be itemPrice × 50000 / 100 instead of 50000

Additional Context

• PR fix(promotion): percentage value is accounted for in buyget promotions #11799 (merged 2025-03-13) fixed percentage support for buyget but did not address fixed amount support. The PR also changed all buyget tests from type: "fixed" to type: "percentage", confirming that fixed was not working.
• The admin dashboard edit form allows changing buyget promotions to type: "fixed", and the value is correctly saved to the database — but the compute engine ignores it.

Environment

• Medusa version: 2.13.1
• @medusajs/promotion version: 2.13.1

Suggested Fix

In buy-get.ts, check promotion.application_method?.type before calculating the amount:

let amount
if (promotion.application_method?.type === "fixed") {
  amount = MathBN.min(
    MathBN.mult(promotion.application_method.value, multiplier),
    applicableAmount
  )
} else {
  const applicablePercentage = promotion.application_method?.value ?? 100
  amount = MathBN.mult(applicableAmount, applicablePercentage).div(100)
}