Reliable Shared Cart Functionality Across Two Shopify Plus Domains

Shopify App Store
1

Hi everyone,

We’re looking for guidance or best practices for implementing a shared cart experience across two Shopify Plus storefronts—specifically, cbcrabcakes.com and mackenzieltd.com.

Goal

We want customers to be able to add products to their cart on one domain and continue shopping or checking out on the other, without logging in. The cart should remain consistent across both storefronts.

What We’ve Tried So Far

Our current approach uses a browser fingerprint-based identifier:

  • A hash is generated client-side (using browser and device traits such as user agent, CPU cores, RAM, screen size, etc.).

  • This fingerprint is sent to our custom Next.js app, which stores and retrieves cart data between the two stores using the Shopify API.

The Problem

This method is unreliable. Fingerprints are not truly unique, and similar devices generate identical hashes, leading to cart collisions (one user seeing another’s cart).

In addition, modern browsers are increasingly restricting access to identifying traits, making fingerprinting even less reliable.

Alternative Consideration

We’re exploring a broker domain + first-party cookie approach:

  • A neutral domain (e.g., id.cff.com) issues a random, anonymous ID to each browser.

  • Both storefronts perform a brief top-level redirect (“bounce”) to this broker on first visit.

  • The broker sets its own cookie and returns the same session ID to both stores, linking them without relying on third-party cookies or fingerprints.

This could technically work, but it introduces potential performance and UX issues (redirect delays, privacy extensions blocking bounces, and added complexity).

What We’d Like to Know

  1. Has anyone successfully implemented cross-domain shared cart functionality between two Shopify Plus stores?

  2. Are there any Shopify-supported or partner-recommended approaches for this (e.g., via app proxy, Multipass login, or server-to-server sync)?

We want to keep the shared cart as part of the experience, but ensure it’s stable and compliant. Any advice or examples would be greatly appreciated.

Thanks so much,

2

Have you tried establishing a postmessage api on both domains that the other domain can talk to? Basically allow an iframe to the other domain to broker cart messages to the domain the customer is currently on.

3

Or this: Channel Messaging API - Web APIs | MDN

4

Isn’t this because you have it backwards instead of having the server do the work to avoid collisions.

If the frontend is the only fingerprint ,
If it’s hidden behind a customer account then the customer ID etc should be in the mix.
If there’s no customer association then how do you even initially connect different devices/websites/sessions with any confidence directly to each other.
Without making the visitor jump through hoops, or with such things as a verification code.
:notebook: yes this usecase seems to be about about a single browser experience just using multiple tabs/windows but the points stand.

That’s not even getting into deciding which site|interaction is authoritative in any overwrites.

Side attack: just embed site A in site B using buy-buttons , or other sdks like the storefront apis

:pie: :cloud: And it’s likely going to evolve past just browsers restrictions, it will be offensive tools generating illusory random traits for people or agents.