WP Remote Payment Client

The problem

A shop needed to accept payment through a gateway that was contracted to a different site. Buyers had to be sent somewhere else to pay, and everything after that was guesswork: the shop had no way to learn the outcome, so an operator checked the other system by hand and moved orders along. Paid orders sat unfulfilled; abandoned ones looked identical to paid ones.

Naive versions of this integration fail in two specific ways. Either the callback endpoint is left open, so anyone who learns the URL can mark any order paid, and the shop believes it. Or the two systems keep separate order records that drift apart, and reconciling them becomes a weekly chore.

Before → After

Before	After
Buyers sent off-site to pay with no way back into the order	A checkout gateway that returns a payment link and follows it through
Payment outcome learned by a person checking another system	Signed callback moves the order to its real status automatically
A callback URL anyone could call	Token-authenticated endpoints with HMAC-signed payloads
Two systems, two order records, weekly reconciliation	A shadow order linked by source and origin id: lookups, not reconciliation
Failures invisible until a customer complained	Every remote exchange written to a log table with status and payload
Wiring a new partner site meant a code change	Endpoints, secret, token and provider set in the gateway settings

How it works

The buyer picks the gateway. It registers as a normal WooCommerce payment method, so it appears at checkout beside every other option and needs no theme work.
A shadow order is created remotely. The order is serialised and sent to the partner site, which creates its own matching order and returns a payment link. The buyer is redirected to it.
Both sides stay addressable.The shadow order carries the origin order id and a source marker, so either system can find the other's record by meta query. There is no separate mapping table to drift.
Payment result comes back signed. The callback endpoint requires a token, and the payload carries an HMAC signature verified against a shared secret with a timing-safe comparison. Only then is the local order status changed.
Every exchange is logged. A dedicated table records the order, the remote order id, the source, the status, the callback URL and the raw payload, so a disputed payment is a query, not an investigation.
Status changes propagate. A handler watches local order status and keeps the remote side informed, so cancelling or refunding on one side does not leave the other stale.

Key features

WooCommerce payment gateway: appears at checkout like any other method
Shadow order creation on a remote site with payment link return
HMAC-SHA256 request signing with timing-safe verification
Token-authenticated REST endpoints for link, lookup and callback
Order matching by source and origin id: no mapping table
Remote exchange log table with status and payload
Order status change propagation back to the remote side
Endpoints, secret, token, provider and payment method all in gateway settings

Installation

Upload the plugin folder to /wp-content/plugins/ and activate it
Run the bundled migration to create the remote order log table
Open WooCommerce → Settings → Payments and enable the external site gateway
Fill in the three remote endpoints, the shared secret, the API token and the provider name
Place a test order and confirm the callback moves it to a paid status