Mollie API integration services (SEPA / OpenFinance)

Deliverables checklist

• OpenAPI 3.1 specification for the wrapper service
• Auth flow report covering API keys, OAuth 2.0 (Mollie Connect) and webhook signatures
• Runnable Python and Node.js source for payments, payment links, statements and webhooks
• Postman collection plus pytest / vitest integration suites against the Mollie test mode
• Operational runbook: idempotency keys, retry strategy, signature verification, webhook backfills
• Compliance note: PSD2, GDPR, SEPA and PCI scope reduction

Two engagement models

You choose how to consume the integration. Both models keep the underlying Mollie credentials with you — we never store production keys.

• Source code delivery from $300 — receive a runnable repository plus documentation; payment is released after on-site validation.
• Pay-per-call hosted API — call our managed gateway and pay only per request, with no upfront fee, ideal for short campaigns or proofs of concept.

1. Accounting & ERP reconciliation

Business context: a Netherlands-based retailer wants Mollie payouts to land in Exact Online or Odoo without manual CSV imports. Data & API: GET /v2/settlements for daily SEPA payout files, joined with GET /v2/payments?settlementId=... per line item. OpenFinance mapping: settlement amount becomes the ledger entry, fee lines map to expense accounts, and the SEPA reference closes the loop with the bank statement coming from PSD2 account information services.

2. Subscription billing with SEPA Direct Debit

Business context: a SaaS vendor in Germany migrates from card-only billing to mandate-backed SEPA Direct Debit. Data & API: create customers, then POST /v2/customers/{id}/mandates with confirmed IBAN, then POST /v2/subscriptions. OpenFinance mapping: mandate IDs and IBANs flow through OpenBanking-aligned PSD2 consents; webhooks announce payment.failed and trigger dunning, keeping retention metrics accurate.

3. Multi-channel checkout with payment links and QR

Business context: a French hospitality group sends invoices over WhatsApp, email, and uses table-side QR. Data & API: POST /v2/payment-links with amount, currency (€, £, CHF, etc. — 25+ supported), and embed the returned URL into a QR. OpenFinance mapping: each settled link becomes a payment record that pipes into the venue's POS, while open balance changes reflect in real time on the manager dashboard.

4. Mollie Capital cashflow scoring

Business context: a finance team running a fleet of 40 e-commerce stores wants to model SME borrowing capacity. Data & API: combine GET /v2/balances, settlements history, and Mollie Capital eligibility/repayment events to produce a 90-day cashflow forecast. OpenFinance mapping: percentage-of-sales repayment data acts as a working-capital signal; the same model can score new merchants for receivables-backed financing.

5. Marketplace splits via Mollie Connect

Business context: a Belgian platform onboards sub-merchants and needs OAuth-based access plus split payments. Data & API: Mollie Connect OAuth 2.0 flow issues a partner access token; subsequent calls to POST /v2/payments include routing data so funds split between platform and seller. OpenFinance mapping: each routed payout becomes an attributable line in the partner's settlement report, useful for payout statements and tax compliance.

Create a payment with webhook (Node.js)

// POST /api/v1/mollie/payments
// Wraps the official @mollie/api-client and returns a normalised object
import { createMollieClient } from '@mollie/api-client';

const mollie = createMollieClient({ apiKey: process.env.MOLLIE_LIVE_KEY });

export async function createCheckout({ amount, currency, description, redirectUrl }) {
  const payment = await mollie.payments.create({
    amount: { value: amount, currency },           // e.g. '49.95', 'EUR'
    description,
    redirectUrl,
    webhookUrl: 'https://your-domain.example/webhooks/mollie',
    method: ['ideal', 'bancontact', 'creditcard']
  });
  return {
    id: payment.id,
    status: payment.status,                        // 'open' | 'paid' | 'failed'
    checkoutUrl: payment.getCheckoutUrl(),
    expiresAt: payment.expiresAt
  };
}

Statement export (Python)

# GET /api/v1/mollie/statement?from=2026-04-01&to=2026-04-30
# Wraps mollie-api-python; pages settlements + payments
from mollie.api.client import Client

client = Client()
client.set_api_key(API_KEY)

def export_statement(from_date, to_date):
    rows = []
    for stl in client.settlements.list():
        if not (from_date <= stl.settled_at[:10] <= to_date):
            continue
        for p in client.settlements.payments.with_parent_id(stl.id).list():
            rows.append({
                'settlement_id': stl.id,
                'payment_id'   : p.id,
                'method'       : p.method,
                'amount'       : p.amount['value'],
                'currency'     : p.amount['currency'],
                'fee'          : p.application_fee.get('amount', {}).get('value'),
                'paid_at'      : p.paid_at,
                'status'       : p.status,
            })
    return rows

Webhook signature verification (Node.js)

// POST /webhooks/mollie
// Mollie next-gen webhooks deliver signed payloads.
// Verify before trusting; reply 200 within 15s, idempotent on payment id.
import crypto from 'node:crypto';

export function verifyMollieSignature(rawBody, signatureHeader, secret) {
  const expected = crypto
    .createHmac('sha256', secret)
    .update(rawBody)
    .digest('hex');
  return crypto.timingSafeEqual(
    Buffer.from(signatureHeader),
    Buffer.from(expected)
  );
}

// Error handling: respond 200 only after persisting the event id;
// otherwise return 5xx so Mollie schedules a retry.

Regulatory alignment

Mollie is a regulated Dutch payment institution supervised by De Nederlandsche Bank, and its Pay by Bank flows are built on the European PSD2 directive. From 9 October 2025, all European banks must support sending Instant SEPA Credit Transfers under Regulation (EU) 2024/886 on Instant Payments — our integrations are designed to consume that real-time flow without code changes for the merchant. We also align to GDPR for personal data processing and to PCI DSS scope reduction by keeping card data inside Mollie's hosted checkout.

Privacy & data minimisation

• API keys live with the merchant; we never store production credentials in our gateway.
• OAuth 2.0 scopes are limited to the data the integration actually consumes.
• Webhook payloads are validated server-side before any database write.
• Personal data such as IBAN, name and email is logged with field-level redaction.
• Retention windows for transactional logs are configurable and default to 13 months.

About OpenFinance Lab

We are an independent technical studio focused on App interface integration and authorised API integration. The team brings hands-on experience from European payment institutions, ERP vendors and protocol-analysis labs, and we ship end-to-end financial APIs under privacy and regulatory constraints.

• Payments, digital banking, lending and cross-border clearing
• Enterprise API gateways, webhook resilience and observability
• Custom Python, Node.js, Go and PHP SDKs and test harnesses
• Full pipeline: protocol analysis → build → validation → compliance
• Source code delivery from $300 — receive runnable API source code and full documentation; pay after delivery upon satisfaction
• Pay-per-call API billing — access our hosted API and pay only per call, no upfront cost; ideal for teams that prefer usage-based pricing

Contact

For quotes or to submit your target app and requirements, open our contact page:

Contact page

Send the target app name, the data fields you need, and any sandbox or partner credentials you already have. We respond with a scoped quote within one business day.

Engagement workflow

1. Scope confirmation: pick the integration scenarios that matter (payments, statements, mandates, Mollie Connect, Capital).
2. Protocol & API design: 2–5 business days, including auth, error model and webhook strategy.
3. Build and internal validation against Mollie test mode: 3–8 business days.
4. Documentation, samples and Postman / pytest suites: 1–2 business days.
5. First delivery: typically 5–15 business days; multi-entity or partner OAuth flows can extend the timeline.

FAQ

Which Mollie data can you actually expose through an API?

Do you use the official Mollie API or do you reverse-engineer the app?

How long does a typical Mollie integration take?

How is compliance with PSD2 and GDPR handled?