[Bug]: Incorrect total and tax amount calculation

[davidmdavis]

Package.json file

{
  "name": "medusa-starter-default",
  "version": "0.0.5",
  "description": "A starter for Medusa projects.",
  "author": "Medusa (https://medusajs.com)",
  "license": "MIT",
  "keywords": [
    "sqlite",
    "postgres",
    "typescript",
    "ecommerce",
    "headless",
    "medusa"
  ],
  "scripts": {
    "build": "medusa build",
    "predeploy": "medusa db:migrate",
    "seed": "medusa exec ./src/scripts/seed.ts",
    "start": "NODE_OPTIONS='-r newrelic' medusa start",
    "dev": "medusa develop",
    "test": "npm run test:unit && npm run test:integration",
    "test:unit": "NODE_OPTIONS=--experimental-vm-modules jest --forceExit --detectOpenHandles ./tests/unit",
    "test:integration": "NODE_OPTIONS=--experimental-vm-modules jest --forceExit --detectOpenHandles ./tests/integration",
    "generate:openapi": "node scripts/generate-openapi.js"
  },
  "dependencies": {
    "@amazonpay/amazon-pay-api-sdk-nodejs": "^2.3.1",
    "@aws-sdk/client-sns": "^3.782.0",
    "@medusajs/admin-sdk": "2.8.4",
    "@medusajs/cli": "2.8.4",
    "@medusajs/framework": "2.8.4",
    "@medusajs/medusa": "2.8.4",
    "@mikro-orm/core": "6.4.3",
    "@mikro-orm/knex": "6.4.3",
    "@mikro-orm/migrations": "6.4.3",
    "@mikro-orm/postgresql": "6.4.3",
    "@opentelemetry/auto-instrumentations-node": "^0.57.0",
    "@opentelemetry/exporter-prometheus": "^0.200.0",
    "@opentelemetry/instrumentation-pg": "^0.52.0",
    "@opentelemetry/resources": "^2.0.0",
    "@opentelemetry/sdk-node": "^0.200.0",
    "@opentelemetry/sdk-trace-node": "^2.0.0",
    "awilix": "^8.0.1",
    "moment": "^2.30.1",
    "newrelic": "^12.16.1",
    "pg": "^8.13.0",
    "prom-client": "^15.1.3",
    "whatwg-fetch": "^3.6.20"
  },
  "devDependencies": {
    "@medusajs/test-utils": "2.8.4",
    "@mikro-orm/cli": "6.4.3",
    "@openapitools/openapi-generator-cli": "2.16.3",
    "@swc/core": "1.5.7",
    "@swc/jest": "^0.2.37",
    "@types/jest": "^29.5.14",
    "@types/newrelic": "^9.14.8",
    "@types/node": "^20.0.0",
    "@types/react": "^18.3.2",
    "@types/react-dom": "^18.2.25",
    "aws-sdk-client-mock": "^4.1.0",
    "jest": "^29.7.0",
    "prop-types": "^15.8.1",
    "react": "^18.2.0",
    "react-dom": "^18.2.0",
    "redis": "^5.0.1",
    "swagger-jsdoc": "^6.2.8",
    "ts-node": "^10.9.2",
    "typescript": "^5.6.2",
    "vite": "^5.2.11",
    "yalc": "^1.0.0-pre.53"
  },
  "optionalDependencies": {
    "@rollup/rollup-linux-x64-musl": "4.34.8"
  },
  "engines": {
    "node": ">=20"
  }
}

Node.js version

v22.14.0

Database and its version

PostgreSQL 16.6

Operating system name and version

MacOS 15.5

Browser name

No response

What happended?

I'm implementing a custom tax provider, but the tax amounts that are being calculated are not being rounded.

When I calculate the taxes on my cart, I'm ending up with the following values:

{
    ...
    "total": 15.8295,
    "original_total": 15.8295,
    "tax_total": 0.8295,
    "original_tax_total": 0.8295,
    ...
}

The following ItemTaxCalculationLine and ItemTaxLineDTO are used to get the above result:

ItemTaxCalculationLine:

{
  "id": "cali_01JKB9YACQGF5N42NN4KESY2YY",
  "product_id": "prod_01JEEHXXE2AE4CJPST6MMFEVG6",
  "product_type_id": null,
  "quantity": 1,
  "unit_price": 15,
  "currency_code": "usd"
}

ItemTaxLineDTO:

{
  "rate_id": "dummy_rate_id",
  "rate": 5.53,
  "name": "Dummy Tax",
  "code": "DUMMY",
  "line_item_id": "cali_01JKB9YACQGF5N42NN4KESY2YY",
  "provider_id": "my-provider"
}

Is it expected that the total amounts and tax amounts are not rounded? Ideally, we would be able to set the tax amount directly instead of a rate similar to the ITaxCalculationStrategy in version 1.x. We have a third party tax provider that calculates the tax amount. If we can only set the rate then that leads to a potential issue where the amount calculated by our tax provider does not match the amount calculated by Medusa. Is there a plan to provide similar functionality for version 2.x?

Expected behavior

Rather than set a tax rate that is used to calculate the tax for a line item each time a cart or order is retrieved, it would be ideal to set the actual tax amount for a line item.

Actual behavior

The tax amounts are calculated based on the set rate when a cart is retrieved. The amounts are not rounded and do match the tax amounts returned by our tax provider.

Link to reproduction repo

N/A

[scherddel]

I don't see an issue here. Why are you saying the total and tax amount are incorrect? This seems misleading.

My calculator shows:

Tax rate: 5.53%
Subtotal: 15

Verification:
Subtotal: 15.0000
Tax amount: 0.8295
Total: 15.8295

Shouldn't it be every developer's freedom to decide how and whether to round values? Every currency works differently, and you have all options available to either round up or down and specify how many digits you want.

[davidmdavis]

I have the decimal_digits value for my currency (USD) set to 2, but it's still not rounded. Is there another way to specify the number of digits? We are also using a third party tax provider that calculates the tax amount. The issue arises if we are only allowed to set the rate, there is the possibility that when the value is rounded it does not match the actual tax amount provided by our tax provider. The ideal solution would be having the option to set tax amount directly instead of having it calculated (and rounded) based on rate.

[scherddel]

To my knowledge decimal_digits does not have any impact on the rounding rules of totals or other values in the cart. I also don't think it makes sense to mathematically interfere with these values, as they always need to be as close as possible to the mathematical source, especially when subsequent calculations are being performed. In your case, I think it makes sense to apply rounding rules at the moment you compare these values with the tax provider in a custom implementation.

[davidmdavis]

The issue is there is no way to correct the tax amount in the current implementation. If the rounded amount calculated by Medusa does not match the amount from our tax provider, we have no way to fix it since it's calculated at runtime when a cart / order line item is retrieved.

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 3 days.

[github-actions]

This issue was closed because it has been stalled for 3 days with no activity.