Connect your systems: API integration without double entry

Shop, accounting, payment providers, CRM: we connect what already runs at your company and let the data flow both ways. If the other side goes down, the integration picks itself up again.

At a glance

1 source of truth

Customer, order, payment: entered once, current everywhere. We agree up front on which system holds the truth.

24/7 in sync

The integration keeps working after hours. Whatever arrives overnight is processed and waiting by morning.

0 duplicate entries

Nobody types the same data a second time. That removes the most common source of errors in daily operations.

100% your accounts

Accounts and keys are registered to your company. Revoke our access any day you like and nothing stops running.

Connecting systems

A pipeline that holds

We build the link between your systems to hold up in daily use: retries when a call fails, clean mapping of the fields and a log that lets you find any single transaction again.

Where system connections fail

Almost every company has good software in house. What's missing is the route from one system to the next.

01

The same data, maintained three times

A new customer gets created in the shop, again in accounting, a third time in the CRM. By the fourth round nobody knows which address is the valid one.

02

Yesterday's numbers

The nightly sync is the one moment in the day when your stock figures are actually right. For the other 23 hours they are an estimate.

03

The connection breaks quietly

A key expires, a field gets renamed, and the transfer stops. Most of the time it surfaces only when a customer asks where their order went.

04

Nobody can look it up

When an order never arrives, the search runs through three interfaces and ends at the contractor who wrote the script two years ago.

What we build into an integration

We combine these building blocks depending on the task. All of it runs in this or a similar form in systems we operate to this day.

01

Connecting systems

Shop, accounting, payment provider, inventory, CRM: we connect anything that has an API, and build one where none exists.

02

Two-way sync

Data flows out and back without one side overwriting the other. For every field it is settled which system is allowed to own it.

03

Reacting the moment it happens

Where the provider supports it, we work with events: the payment reports itself, long before the nightly sync ever runs.

04

Connected securely

Incoming messages are checked against their signature, keys are stored encrypted, and every write carries an identifier against double bookings.

05

Clean field mapping

Customer number, tax rate, unit: two systems rarely mean the same thing. We keep the mapping documented and changeable, with no need to touch code.

06

Your own API for partners

When others need to query your data, we build the API for it: documentation, access keys and versioning that keeps old calls alive.

07

Legacy systems & bulk imports

Existing data from spreadsheets, exports or a system being retired comes over in one migration, in stages and with a dry run before the real thing.

08

Visibility into every transfer

Every transfer lands in a log with timestamp, result and payload. When something gets stuck, you see it there before a customer calls about it.

09

Operations & upkeep

Providers change their APIs, keys expire. We monitor the pipelines, follow the version changes and get in touch before something stalls.

Systems we have already connected

Each of these integrations is code we wrote for projects we run today.

01

Payments & billing

  • Stripe
  • PayPal
  • Credit cards
  • SEPA Direct Debit
  • Apple Pay
  • Google Pay
  • Klarna

The payment methods run through Stripe or through your own payment provider. The interesting work sits below the surface: one authorization from which deposit and balance are later collected separately, subscription billing with direct debit and automatic tax, and the rule that an amount counts as paid once the provider's signed callback confirms it. Whether your books balance at month end hangs on exactly that.

02

Credit, identity & tax reporting

  • Credit checks (SCHUFA in Germany)
  • Stripe Identity
  • VAT ID validation (VIES)
  • VeriFactu (Spain)

Before an inquiry turns into a contract, it often has to be established who is actually signing. Credit and solvency checks against a credit bureau run in production for us: the German SCHUFA connection goes over the signed XML interface with its own participant certificate, including the cases a person has to review and answer inside a deadline. The bureau that counts in your market gets connected the same way. VAT ID validation decides tax rate and invoicing scheme. For the Spanish market, the VeriFactu reporting obligation is built and tested.

03

Shop & commerce systems

  • Shopware 6 (Store API & Admin API)
  • Shopware 5 (in-house plugin)
  • License management
  • S3 & CloudFront

Selling licenses or downloads means the work carries on past the order. A license system runs alongside the shop: it issues the keys for a paid order and checks stock before anything is handed out. The files sit in protected storage and are released only for the order they belong to.

04

Booking, calendars & portals

  • Channel managers
  • iCal calendars
  • Google Hotel Center
  • Google Vacation Rentals

Anyone offering rooms, appointments or vehicles across several portals needs one calendar that all of them follow. Portals like Booking.com and Airbnb are reachable through a channel manager or through calendar feeds. The return direction is the delicate part: an imported entry must never travel back out as your own, and a departure and an arrival on the same day have to stop blocking each other. For Google we also built the supplier side, which delivers property data and answers price queries at the moment of the search.

05

Email, WhatsApp & push

  • Mailchimp
  • Resend
  • SMTP
  • Gmail API
  • WhatsApp Business
  • Web Push

A single case rarely concerns one person. We build dispatch so that one status change reaches the customer, the partner and the back office at the same time, each on the channel they agreed to: approved WhatsApp templates, Web Push or plain email. Newsletters run with double opt-in and provable consent. The dispatch service behind it stays swappable.

06

AI & translation

  • OpenAI
  • Anthropic
  • DeepL
  • Mistral
  • EU-hosted models
  • dragoman

Answers appear word by word while the model is still writing. The content guardrails sit on the server side, which keeps an assistant from saying what it is legally not allowed to say. Where the processing happens is a decision of its own: for personal content, a model hosted in the EU can take the job. Translations run through dragoman, our open-source tool, where only new and changed text costs anything.

07

Data, maps & locations

  • Supabase
  • Firestore
  • Mapbox
  • Google Maps Geocoding

Map search with a result list and a radius, address autocomplete while someone is still typing, and database functions for records that two people must never grab in the same moment. Location data is rarely the headline of a project and turns up in almost every one.

How your integration comes to life

Four steps from taking stock to running operations.

01

Systems & data paths

We look at which systems you run in house, which data should travel between them and which system wins when two of them disagree. The result is a diagram your accountant can follow.

02

Concept & fixed price

You get the field mapping, the direction of every sync and the error handling in writing, plus a fixed price per stage. You know what will be built before the first line of code.

03

Integration & trial runs

We build against the provider's sandbox and play through real cases, awkward ones included: duplicate notification, timeout, remote system offline. Only then does it touch live data.

04

Go-live & operations

The first pipeline goes live and takes work off your team at once, the rest follow in the order that pays off most. We then watch the connections and adapt to provider changes.

Ready?

Which two systems should be talking?

Name the two programs with the most manual work running between them. In a free consultation we tell you whether an API exists for the job and roughly what the integration costs.

Frequently asked questions about APIs and integrations

A single integration usually starts at around €4,000, and larger projects spanning several systems land between €10,000 and €40,000. Three things decide the number: how many systems are involved, which direction the sync runs and how good the API on the other side is. A documented modern API costs considerably less than a legacy system we have to pry the data out of. After the first conversation you get a fixed price per stage.

Yes, that's the normal case. We read our way into the documentation and build against the provider's sandbox. When there is no API at all, we look at the detours: scheduled file exports, database access or an import via spreadsheets. Before the quote you get an honest read on how workable the other side really is.

We plan for that from the start. Failed transfers move into a queue and get retried at growing intervals, so a short outage costs no data. Every retry carries an identifier that keeps the same record from arriving twice. If a pipeline stays down for good, an alert goes out the same day.

As often as the process needs. Where the provider sends events, the integration reacts within seconds: a payment reports itself the moment it clears. Where events are missing, we poll on a fixed rhythm, anywhere from every few minutes to once a night. Each pipeline gets its own interval, because stock levels and master data carry different levels of urgency.

You do. Payment provider accounts, API keys and certificates are registered to your company, and we work with our own logins underneath them. When the collaboration ends, you revoke our access and everything keeps running. The same applies to the integration code together with its documentation.

That happens regularly, usually with notice. Which is why we stay on the pipelines after go-live: we follow the change announcements from the providers, move versions forward and test against the new release beforehand. When your day-to-day never notices any of it, the job was done well.

Yes, and we do it more often than you'd think. We cut the API contract-first, keep the specification versioned in the project and let old calls keep working when a new release appears. On top come access keys per partner, a limit on the number of queries and documentation your partner can start from without a single follow-up question. On platforms we run today, third parties query property data and prices exactly this way.

Incoming messages are checked against their signature before they trigger anything; otherwise anyone could fake a paid invoice. Keys and certificates are stored encrypted and away from the code, and test and production environments stay cleanly apart. Only the data the purpose requires gets transferred, and where personal data is involved, we check the place of processing beforehand.

You decide. We monitor the pipelines, handle the changes coming from providers and build the next integrations; many clients have worked with us this way for years. Or we hand over to your IT team, with documentation, field mapping and onboarding. Both are normal paths.

Get in Touch

Is something still running by hand between two of your programs? Tell us which ones they are. We'll tell you whether the two can be connected and roughly what that costs.