Klaviyo Champions Ross Hopkins and Christian Norbjerg Enger share when and why to use APIs. From sending custom events and enriching profiles to automating workflows at scale, their real-world ecommerce examples show how APIs unlock the flexibility and personalization you can’t get from the UI alone.

Why bother with APIs if Klaviyo already has a robust UI?

Ross: 

For me, it's all about flexibility and speed. The UI is great, but sometimes you just want to do something your own way - especially if you're working with internal systems that don’t fit into the mold of out-of-the-box integrations or tools like Zapier and Make.

At CFI, we track the performance of a 12-month membership subscription, and a lot of that logic just doesn’t live in Klaviyo natively. And that’s totally fine, that’s exactly what APIs are for - to provide the control to define the metrics that matter to our business - not just the ones that third-party platforms say I should care about.

Plus, once you start learning how the APIs work, you suddenly get better at using tools like Make and N8N anyway. You start understanding the "why" behind the workflow.
 

Christian:

For me I think that the UI gets you 80% of the way. APIs unlock the last 20% that makes your customer journey unique. With Klaviyo APIs you can 1) send custom events from third-party softwares, checkout, or subscriptions with an open API to trigger flows your CMS platform doesn’t send by default, 2) enrich profiles with zero/first-party data at the moment it’s captured (Like a Checkout URL, last purchase product, profit on order and so much more), and 3) automate bulk jobs (Profile deletion, list/segment operations) without manual work. 

What are your top real-world use cases for APIs in ecommerce marketing?

Ross:

One practical example: I used the API to speed up how we build location and timezone-based segments. We have a global audience, and the requirement was to build segments for multiple regions. Using the API - and a little GenAI to generate region lists - saved me hours of manual work and let us launch targeted campaigns far quicker.

At a larger scale, we’ve used the API to fully track our customers’ subscription journey. From signup, through plan changes, renewals, and course engagement - every key moment is pushed into Klaviyo. That means the customer journey is something we can actually see, respond to, and personalize around.
 

Christian:

One practical example: I used the create/update API in Klaviyo to save the customers checkout URL, so the checkout URL link can be included into a banner earlier in the funnel. Using the Checkout URL, we can send the customer back to their checkout without the need to wait for them to return to checkout and trigger our Abandoned checkout flow. The following is the webhook script, if you want to use it: 

{

  "data": {

    "type": "profile",

    "attributes": {

      "properties": {

        "Checkout URL": "{{ event.extra.checkout_url|default:'' }}"

      },

      "email": "{{ person.email|default:'' }}"

    }

  }

}

And this is the API: https://developers.klaviyo.com/en/reference/create_or_update_profile

At a larger scale across our clients, we’ve used the API’s both in Klaviyo and outside of Klaviyo (Via. Shopify Flow) or Make.com or N8N to pull data from the clients software stack to populate the customer properties with data and create new events. That could be “Meta Ads tracked” to do dynamic retargeting on Meta Ads clicked.

Youtube tutorials on the uses Ross and Christian mentioned 👇

Webhooks sound technical… how do you actually use them in flows?

Ross:

Funnily enough, I mostly use them to move data around in Klaviyo - not just send it out.

I’ve built “data admin” Flows that take info from an event or metric and write it to a profile property. That means I can personalise a campaign email based on someone’s certification interests, or build a Flow that branches based on that property later. It’s like giving Klaviyo memory.

I’ve also used webhooks to push Klaviyo data out to other platforms - so everything stays in sync and nothing is left siloed. The goal is always the same: one ecosystem, one source of truth, regardless of the tool.
 

Christian:

I totally understand why it can seem confusing. 

Webhooks in the most simplest sense is: A way for one system (like Klaviyo) to automatically send data to another system in real time. Think of it as a “live data message” triggered by an event. A webhook has an endpoint: Imagine the specific URL like the address of the data. Each webhook has headers, which are static extra instructions included with the request so the receiving system knows how to handle it. 

In short: webhook = the message, endpoint = the address, headers = the rules that let the systems talk properly.

I mainly use webhooks in Klaviyo flows to push the platform beyond just email, SMS, or reviews. Instead of seeing a flow as only a sequence of messages, I treat it as an automation hub that can both send and receive data across systems.

For example, a webhook can call an external API mid-flow to enrich customer data like pulling real-time shipping updates, loyalty balances, or even weather forecasts. Using Make.com, the flow sends customer info to a webhook, Make fetches the extra data, and then pushes it back into Klaviyo through the Update/Create Profile API. This means the next email or segment decision is based on live data, not static profile fields.

I also use webhooks to capture and store things like checkout URLs, cart contents, or custom event tags. These can then be reused later in automations, whether for personalization, reminders, or segmentation.

So while webhooks may sound technical, in practice they simply let flows connect systems and data. The result: Klaviyo becomes the hub of the customer journey, not just a tool for sending messages

What’s a simple “first project” you’d recommend for someone new to APIs?

Ross:
Try using a webhook to copy one useful event detail - like a product viewed, blog topic, or product category - into a profile property.

It’s simple, but it unlocks so much. Instead of using event-triggered data only in that moment, you now have something persistent you can use across campaigns, Flows, and segmentation. 

The idea is that you’re building trust signals. You’re not guessing what someone’s interested in, you’re using Klaviyo to remember.

Christian:

A great first project is to use an API to enrich customer data in Klaviyo. For example, set up a webhook in a flow that sends a customer’s email to a simple external API like one that returns their current local weather or time zone. Then, write that data back to their profile in Klaviyo

What’s the biggest mindset shift marketers need to make when working with APIs?

Ross:

I’d say “drop the fear”. I know that I had to. I thought they were only for developers to use.

APIs exist to give you flexibility, and once you stop thinking of them as “too technical,” you realize how empowering they are. You don't need to be a developer to use them meaningfully. You just need curiosity and a use case.

Plus, the more you explore APIs in one platform, the more you start understanding how most marketing platforms work. That’s a real skill that is going to set you apart and make you more creative in the future.

Christian:

As Ross mentioned, “Drop the fear”. APIs aren’t “just for developers” Instead of thinking “what does Klaviyo (or Shopify, or Meta or another third-party software) let you do out of the box,” start thinking “what data do I need, and where can I pull or push it?”

Once you see APIs as blocks, the mindset shifts from tool-limited to system-wide. You’re no longer fixed about what you can do inside of Klaviyo. By the API’s you can connect platforms, enrich customer profiles, and trigger actions anywhere.

In short: stop thinking in terms of features in Klaviyo, start thinking in terms of system-wide thinking. 

What’s your favorite API-driven project you’ve built in Klaviyo, and how did it change the way you work?

Ross:

​​It has to be the data copy process - taking metric-level data and saving it to profiles.

At CFI, our “product” is online certification and learning content. So we’ve built a way to track interest in specific certifications, topics, and career paths. That data lives both at the event level and as profile properties, which means we can use it in campaigns, Flows, segmentation, and when building reports.
 

Christian:

Really great question! I think my favorite API-driven project, was to match weather data with peoples city-property on their property to push the right campaign topic to the right people in regards to the weather in their city. We used an open weather API and custom actions and the create/update API in Klaviyo.

Looking ahead, how do you see APIs playing a role in the future of ecommerce marketing, especially with Klaviyo’s new features rolling out this fall?

Ross:

I hate to be boring, but I have to mention AI, right?

I’m really keen to experiment with Klaviyo’s MCP (Marketing Communications Platform) capabilities. Having built a lot of the data foundation manually, I feel more confident experimenting with these new AI tools - because I know how the data is structured, and I trust where it’s coming from.

I’m also excited about Custom Actions, which may actually flip my approach on its head a little; currently, we use API’s to send data into Klaviyo, but with Custom Actions, we’re likely to start reaching out and ask external systems for useful data at a particular point in time.
 

Christian:

For me, the real future of APIs in ecommerce marketing is about orchestration. Right now, most brands think in terms of “syncing” data into Klaviyo. But the next step is to have flows actively query other systems at the exact moment they need context from other softwares and backends.

With Klaviyo rolling out Custom Actions this becomes much more accessible. Instead of preloading all possible data, we can pull what we need on demand. Combine that with MCP via. Claude or Cursor, you get AI tools that are only as strong as the data foundation you build. 

Closing thoughts

APIs may sound technical, but they’re really about creating flexibility—helping you enrich customer profiles, connect systems, and automate the last 20% of the customer journey that the UI can’t cover on its own. The Champions’ examples show how even simple API projects can save hours, unlock personalization, and future-proof your marketing stack