API, webhooks and Zapier

The Quillstack REST API lets you create clients, invoices and payments from your own systems. Create a key in Settings → Developers → API keys. Keys are scoped: read-only or read-write. The rate limit is 120 requests per minute per key. API access is included in the Studio plan.

Add an HTTPS endpoint in Settings → Developers → Webhooks and choose events, for example invoice.sent, invoice.paid, payment.failed and client.created. Each delivery is signed with an HMAC-SHA256 signature in the Quill-Signature header. Failed deliveries are retried with exponential backoff for up to 24 hours.

The Zapier app supports triggers for new invoices, paid invoices and new clients, and actions to create a client or an invoice. It is available on Solo and Studio plans.

Every Studio account has a sandbox workspace with test API keys. Payments in the sandbox use test cards and never move real money.