Square payments

Accept Square payments on invoices alongside Stripe and/or PayPal. When Square is enabled, the public invoice pay page shows a Pay $amount button (Pay with Square when more than one provider is on); the customer pays on Square's hosted page and the payment records in SlowBooks with the standard journal entries (DR Undeposited Funds / CR Accounts Receivable, or the clearing account and fee when Post to a clearing account is on, see Stripe payments, "How it posts").

See Stripe payments and PayPal payments for the other providers — any combination can be enabled. The notes there on who can change these settings, where the pay link works and USD-only invoices apply to Square too.

Prerequisites

• A Square account (squareup.com)
• Slowbooks Pro running and accessible

Step 1: Get sandbox credentials

1. Log in to the Square Developer Console
2. Create an application (e.g. "SlowBooks")
3. On the app's Credentials page, with Sandbox selected at the top, copy the Sandbox Access Token; on its Locations page, copy a sandbox Location ID

Step 2: Configure Slowbooks

1. Open Settings → Online Payments → Square (an administrator's setting)
2. Set Square Payments to Enabled and Environment to Sandbox
3. Paste the Access Token and Location ID
4. Save

Step 3 (server installs): Add a webhook

Skip this on desktop installs — webhooks can't reach 127.0.0.1; payments record when the customer returns from Square, or via the Check Payment Status button on the invoice. (On a desktop or LAN install the pay link itself only opens on your own network, see the Stripe guide.)

1. In the Developer Console: your app → Webhooks → Subscriptions → Add subscription
2. URL: https://your-server/api/payments/square/webhook (HTTPS; Square refuses plain HTTP)
3. Choose an API version and subscribe to payment.updated
4. Copy the subscription's Signature Key into Slowbooks
5. Important: paste the exact same URL into Slowbooks' Webhook Notification URL field — Square signs each delivery over that exact URL string, so verification fails if they differ (e.g. behind a proxy).

Step 4: Test in sandbox

1. In Slowbooks, open an unpaid invoice → Copy Payment Link → open in a private window
2. Click Pay $… (or Pay with Square) and pay with the sandbox test card 4111 1111 1111 1111, CVV 111, any future expiry and a valid ZIP such as 94103 (a USD sandbox payment needs a real-looking postal code; CVV 911 forces a decline)
3. On return you should see "Payment received — thank you!"
4. Verify: invoice Paid, payment row (method square), journal entry posted

Going live

Switch to the app's Production tab, copy the production Access Token and a real Location ID, set Environment to Production, and (server installs) recreate the webhook subscription in production mode with its own signature key.

Notes

• Amounts are charged in USD for the invoice's balance due at checkout time.
• Square payment links carry no metadata, so SlowBooks matches payments to invoices by the order id stored when the checkout was created. Each press of Pay makes a new Square checkout and only the latest order id is kept, so a payment made on an older Square page is answered "invoice not found" and not recorded: record that one by hand.
• Refunds are issued from the Square dashboard. In SlowBooks, void the online payment for a full refund (the posting reverses, through the clearing account when that is on) or enter a credit memo for a partial one.