GlobalBill Multi-Currency Invoicing

Business

Original Idea

Multi-Currency Invoicing A web app that applies live FX rates, local tax templates, and localized invoices.

Product Requirements Document (PRD): GlobalBill Multi-Currency Invoicing

1. Executive Summary

GlobalBill is a next-generation SaaS invoicing platform designed for the 2026 international business landscape. It automates the complexity of multi-currency billing by integrating real-time FX rates, localized tax compliance engines (VAT/GST), and dynamic PDF generation. By utilizing a "Three-Currency" ledger model, GlobalBill allows businesses to bill in any currency while maintaining crystal-clear internal reporting in their base currency, effectively neutralizing the "currency noise" of global trade.

2. Problem Statement

International businesses lose an estimated 3-5% of revenue annually due to:

  • Manual FX Calculation Errors: Using outdated or "google-searched" rates that don't reflect actual settlement values.
  • Tax Non-Compliance: Failure to adhere to shifting global mandates like the EU’s "VAT in the Digital Age" (ViDA).
  • FX Volatility: Lack of visibility into realized vs. unrealized gains/losses, making financial forecasting difficult.
  • Localized Friction: Invoices that aren't formatted to local standards (date formats, currency symbols) lead to delayed client payments.

3. Goals & Success Metrics

  • Goal 1: Reduce invoice generation time for international clients by 70%.
  • Goal 2: Eliminate manual tax calculation errors via API-first compliance.
  • Goal 3: Provide 100% visibility into FX impact on revenue.

Success Metrics:

  • North Star: Time elapsed from "Invoice Created" to "Payment Settled."
  • Accuracy: Zero discrepancies between billed tax and reported tax during audits.
  • Retention: Percentage of users utilizing more than 3 distinct billing currencies.

4. User Personas

  • Freelancer Fiona: Works for clients in the US, EU, and UAE. Needs a simple way to send professional, localized invoices without being an expert in VAT.
  • Agency Alex: Managing Director of a 20-person consulting firm. Needs high-level "Revenue Bridge" reports to see how currency fluctuations affect their bottom line.
  • SaaS Sam: Founder of a global software startup. Needs automated recurring billing that handles localized tax for 100+ countries via Stripe.

5. User Stories

  • As a Freelancer, I want to select a client's country and have the app automatically suggest the correct tax rate and currency, so that I don't send an illegal invoice.
  • As a Financial Controller, I want to see a "Waterfall Chart" of my revenue, isolating the impact of FX fluctuations, so that I can explain revenue drops to stakeholders.
  • As a Client, I want to receive an invoice in my local language and currency with a "Pay Now" link that supports my local payment methods (e.g., SEPA, iDEAL), so that I can pay quickly.
  • As an Admin, I want the system to fetch the spot rate at the moment of invoice issuance and "freeze" it, so that my audit trail is immutable.

6. Functional Requirements

6.1. Core Invoicing & FX Engine

  • Real-time FX Synchronization: Integration with Open Exchange Rates API.
  • Three-Currency Ledger: Every transaction must record:
    1. Transaction Currency (TC): What the client sees.
    2. Functional Currency (FC): The business's local book currency.
    3. Reporting Currency (RC): For consolidated global reporting.
  • Spot Rate Freeze: The exchange rate must be captured at the issued_at timestamp and stored permanently on the invoice record.

6.2. Localized Tax Engine

  • Automated VAT/GST/Sales Tax: Integration with Stripe Tax or Kintsugi to calculate taxes based on the client's tax jurisdiction and product type.
  • Tax ID Validation: Real-time VIES/local database validation for B2B tax-exempt transactions.

6.3. Document Generation

  • Dynamic PDF Engine: Using Puppeteer to render HTML-to-PDF, supporting text expansion (e.g., German translations) and Unicode characters.
  • Intl-Standard Formatting: Automated formatting of dates (DD/MM/YYYY vs MM/DD/YYYY) and currency symbols ($100 vs 100 €) based on BCP 47 locale tags.

6.4. Reporting & Analytics

  • FX Bridge Analysis: Dashboard visualizing the bridge from "Constant Currency Revenue" to "Reported Revenue."
  • Realized vs. Unrealized Tracking: Automatic calculation of G/L when a payment is marked as settled at a different rate than the issued date.

7. Technical Requirements

7.1. Tech Stack (2026 Standards)

  • Frontend: React v20.0.x (patched for CVE-2025-55182), TypeScript 5.7+, Tailwind CSS v4.1.18 (Oxide engine).
  • Backend: NestJS v11.1.12 running on Node.js 22 (LTS).
  • Database: PostgreSQL 18 (utilizing native UUIDv7 and TSTZRANGE for temporal exchange rates).
  • Caching: Redis (via cache-manager-redis-yet) for L2 caching of FX rates.
  • PDF Generation: Puppeteer (Headless Chrome) for HTML-to-PDF rendering.

7.2. Critical Integrations

  • Financial Data: Open Exchange Rates API (with SWR caching strategy).
  • Payments & Tax: Stripe API (2026 version) using "Multi-currency Settlement" and "Payment Records API."
  • Email: SendGrid for localized invoice delivery.

8. Data Model

8.1. Entities

  • Organization: id (uuidv7), name, base_currency (char3), tax_id.
  • Client: id (uuidv7), org_id, name, email, default_currency, tax_region (iso_country_code).
  • ExchangeRate: id (bigint), pair (char6), rate (numeric 18,8), validity (tstzrange).
  • Invoice:
    • id (uuidv7)
    • billed_amount (numeric 19,4)
    • billed_currency (char3)
    • base_amount (numeric 19,4)
    • applied_exchange_rate (numeric 18,8)
    • status (draft, issued, paid, void)
  • LineItem: id, invoice_id, description, quantity, unit_price, tax_rate_applied.

9. API Specification (Key Endpoints)

POST /api/v1/invoices

Request Body:

{
  "clientId": "uuid",
  "items": [
    { "description": "Consulting", "qty": 10, "unitPrice": 150.00 }
  ],
  "currency": "EUR",
  "issueDate": "2026-02-01T10:00:00Z"
}

Response: 201 Created with calculated base_amount and tax_total.

GET /api/v1/reports/fx-impact

Query Params: startDate, endDate, baseCurrency. Response: Data for waterfall chart (Volume, Price, FX Fluctuation).

10. UI/UX Requirements

  • Currency Switcher: A global toggle to view the entire dashboard in "Constant Currency" vs "Actual Rates."
  • Invoice Preview: A side-by-side view showing the "Client View" (Localized PDF) and "Accounting View" (Base currency breakdown).
  • Data Visualization: Use Recharts for the Revenue Bridge (Waterfall chart).

11. Non-Functional Requirements

  • Security: AES-256 encryption for all tax-related identifiers. Implementation of the December 2025 React security patch to mitigate "React2Shell" RCE.
  • Performance: <100ms for FX conversion lookups (via L1 memory cache).
  • Compliance: Support for Peppol e-invoicing formats required in the EU by 2026.

12. Out of Scope

  • Full-scale General Ledger (GL) accounting (Integration with Xero/QuickBooks only).
  • Cryptocurrency custody (Payments accepted via Stripe/Crypto.com, but converted to fiat).
  • Employee payroll processing.

13. Risks & Mitigations

  • Risk: API Downtime for Open Exchange Rates.
    • Mitigation: 24-hour fallback to the last successful "Frozen" rate in PostgreSQL and circuit breaker implementation.
  • Risk: Rapid Tax Law Changes.
    • Mitigation: Outsource calculation logic to Stripe Tax/Anrok rather than hardcoding rates.
  • Risk: React2Shell Vulnerability.
    • Mitigation: Strict CI/CD linting to ensure no React v20 builds are deployed without the December 2025 security patches.

14. Implementation Tasks

Phase 1: Project Setup

  • [ ] Initialize NestJS v11.1.12 with Node.js 22 LTS.
  • [ ] Initialize React v20.0.x with Tailwind CSS v4.1.18.
  • [ ] Configure PostgreSQL 18 with UUIDv7 extensions.
  • [ ] Set up Docker orchestration for local Redis and Postgres instances.

Phase 2: Core FX & Ledger

  • [ ] Create exchange_rates table with TSTZRANGE temporal constraints.
  • [ ] Implement ExchangeRateService with 1-hour Redis caching.
  • [ ] Build the "Three-Currency" ledger logic in the Invoice Creation service.
  • [ ] Implement the calculation for Realized vs. Unrealized gains on payment settlement.

Phase 3: Tax & Document Generation

  • [ ] Integrate Stripe Tax API for real-time VAT/GST calculation.
  • [ ] Build HTML invoice templates using Tailwind CSS.
  • [ ] Implement Puppeteer service for localized PDF generation with i18next support.
  • [ ] Add font subsetting for global scripts (CJK, Arabic).

Phase 4: Reporting & UI

  • [ ] Create Dashboard UI with Waterfall charts for FX Impact analysis.
  • [ ] Build Client Directory with default currency/tax profile settings.
  • [ ] Implement "Payment Link" generation using Stripe's dynamic localization.
  • [ ] Final security audit for "React2Shell" (CVE-2025-55182) compliance.