Nexus and FastSpring embedded checkout integration

Use FastSpring with Nexus to support creator-code attribution at checkout. Buyers enter a creator's code during purchase, FastSpring optionally applies a discount, and your server records the attribution in Nexus so the creator gets credit. FastSpring acts as the merchant of record for all transactions.

This page covers a working reference implementation from the FSBuilds GitHub repository, found in the fsbuilds-nexus-integration folder of the FastSpring fsBuilds examples repo. You can download it, configure your credentials, and have a functioning integration running locally in minutes.

How it works

Your server acts as the bridge between FastSpring and Nexus — the two platforms never communicate directly. This keeps your Nexus private key off the browser and gives you full control over the flow.

sequenceDiagram
    participant B as Buyer
    participant F as Frontend
    participant S as Your Server
    participant N as Nexus API
    participant FS as FastSpring

    rect rgb(240, 253, 244)
        note over B,N: 1 — Code validation
        B->>F: Enters creator code
        F->>S: POST /validate-code
        S->>N: Validate (public key)
        N-->>S: Creator name or error
        S-->>F: Validation result
        F-->>B: Creator message or error
    end

    rect rgb(255, 251, 235)
        note over F,FS: 2 — Discount application (optional)
        F->>FS: Apply coupon code
        FS-->>F: Discount applied
    end

    rect rgb(245, 243, 255)
        note over B,FS: 3 — Purchase & attribution
        B->>FS: Completes purchase
        FS-->>B: Order confirmation
        FS->>S: Webhook: order.completed
        S->>N: POST attribution (private key)
        N-->>S: Creator credited ✓
    end

The three phases in plain language: validate the code before purchase using your public key, apply a discount if the code matches a FastSpring coupon, and record the attribution after purchase using your private key.

Creator code states

A code entered at checkout can match Nexus, FastSpring, both, or neither. The following table shows what happens in each case.

Prerequisites

Before you begin, make sure you have the following.

Set up the integration

Project structure

The reference implementation has a minimal footprint — a frontend file, a backend file, and configuration.

fsbuilds-nexus-integration/
├── index.html       # Frontend — embedded checkout, creator code input, product display
├── server.js        # Backend — webhook handler, Nexus API calls
├── style.css        # Styles
├── .env             # Your credentials (never committed to Git)
├── .env.example     # Template showing which environment variables are required
└── package.json     # Node dependencies

The two files you will spend time in:

Test the integration

Before going live, verify your setup using FastSpring's sandbox environment.

Security

Related resources

• Nexus API documentation
• Nexus key documentation
• FastSpring Store Builder Library reference
• FastSpring Webhooks reference
• FastSpring embedded checkout setup
• FSBuilds GitHub repository