Invoicing API — Free digital invoices with email delivery

Features

Professional invoicing without the accounting software complexity.

Auto-numbered invoices

Per-tenant sequence with configurable prefix. Zero-padded 5-digit numbers like INV-00001.

Line items and tax

Add multiple line items with description, quantity, and unit price. Optional flat tax applied to subtotal.

HTML print view

Server-side rendered HTML invoice with clean typography. Open in browser and print to PDF.

Email delivery

Send invoices directly to customers via email. Configure a sendTo address per invoice.

Multi-currency

Set currency per invoice (default EUR). All amounts stored in cents for precision.

CSV export

Export all invoices as CSV with line items, totals, and status for accounting.

Use cases

Any business that needs to issue invoices can use NexusKit Invoicing.

Freelancers and contractors

Issue professional invoices to clients with auto-numbering and email delivery.

Small businesses

Generate invoices for products or services without accounting software overhead.

SaaS billing

Create invoices for custom plans, add-ons, or one-time charges alongside your subscription system.

Agency billing

Invoice clients for projects with detailed line items and tax calculation.

How the API works

Create an invoice, get a printable HTML view, and optionally email it to your customer.

1. Create an invoice

POST /v1/invoices
{
  "buyer": {
    "name": "Acme Corp",
    "email": "[email protected]"
  },
  "lineItems": [
    { "description": "Website design", "quantity": 1, "unitCents": 50000 },
    { "description": "Hosting (12 months)", "quantity": 12, "unitCents": 1500 }
  ],
  "currency": "EUR",
  "taxCents": 10300,
  "sendTo": "[email protected]"
}

2. Get the print view

GET /v1/invoices/:id/print
// Returns text/html — open in browser and print to PDF

3. Export for accounting

GET /v1/invoices/export.csv
// Returns text/csv with all invoices

FAQ

How does auto-numbering work?

Each tenant gets a sequence starting at 00001. The prefix defaults to INV but is configurable. Numbers are zero-padded to 5 digits (INV-00001, INV-00002, etc.).

Is NexusKit Invoicing free?

Yes, within fair-use limits. No monthly caps on invoice creation — just rate limits on the API (120 RPM).

Can I send invoices by email?

Yes. Set the sendTo field when creating an invoice. The HTML invoice is sent via the mail queue automatically.

What does the print view look like?

Clean, professional HTML with Georgia serif font, styled table layout, and formatted currency. Open in browser and print to PDF.

Can I customize the invoice template?

The current template is a clean, professional default. Custom templates are on the roadmap.

Invoices