Automate Invoice Signing with an API: From Quote to Signed in 30 Seconds

interface LineItem {
  description: string;
  quantity: number;
  rate: number;
}

function generateInvoice(params: {
  invoiceNumber: string;
  clientName: string;
  clientEmail: string;
  issueDate: string;
  dueDate: string;
  items: LineItem[];
}) {
  const subtotal = params.items.reduce(
    (sum, item) => sum + item.quantity * item.rate, 0
  );
  const tax = subtotal * 0.2; // 20% VAT
  const total = subtotal + tax;

  const itemRows = params.items
    .map(
      (item) =>
        `| ${item.description} | ${item.quantity} | $${item.rate.toFixed(2)} | $${(item.quantity * item.rate).toFixed(2)} |`
    )
    .join("\n");

  return `
# Invoice ${params.invoiceNumber}

**From:** YourCompany Ltd
**To:** ${params.clientName}
**Issue Date:** ${params.issueDate}
**Due Date:** ${params.dueDate}

---

## Line Items

| Description | Qty | Rate | Amount |
|-------------|-----|------|--------|
${itemRows}

---

| | | **Subtotal** | **$${subtotal.toFixed(2)}** |
|--|--|--|--|
| | | **VAT (20%)** | **$${tax.toFixed(2)}** |
| | | **Total Due** | **$${total.toFixed(2)}** |

## Payment Terms

Payment is due within 30 days of the issue date.
Late payments are subject to 1.5% monthly interest.

By signing below, you confirm receipt of this invoice
and agree to the payment terms stated above.
`;
}

async function sendInvoiceForSigning(invoice: {
  invoiceNumber: string;
  clientName: string;
  clientEmail: string;
  issueDate: string;
  dueDate: string;
  items: LineItem[];
}) {
  const markdown = generateInvoice(invoice);

  const response = await fetch("https://signb.ee/api/v1/send", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "Authorization": "Bearer sk_live_your_api_key",
    },
    body: JSON.stringify({
      markdown,
      recipient_name: invoice.clientName,
      recipient_email: invoice.clientEmail,
      subject: `Invoice ${invoice.invoiceNumber} — Please review and sign`,
    }),
  });

  const { id, status } = await response.json();

  // Store the document ID against the invoice
  await db.invoices.update({
    where: { number: invoice.invoiceNumber },
    data: { signbeeDocId: id, signingStatus: status },
  });

  return { id, status };
}

import { NextRequest, NextResponse } from "next/server";
import Stripe from "stripe";

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!);

export async function POST(req: NextRequest) {
  const payload = await req.json();

  if (payload.event === "document.signed") {
    // Find the invoice linked to this document
    const invoice = await db.invoices.findFirst({
      where: { signbeeDocId: payload.document_id },
    });

    if (!invoice) return NextResponse.json({ error: "Not found" }, { status: 404 });

    // Create a Stripe payment link
    const session = await stripe.checkout.sessions.create({
      mode: "payment",
      line_items: [{
        price_data: {
          currency: "usd",
          unit_amount: Math.round(invoice.totalAmount * 100),
          product_data: {
            name: `Invoice ${invoice.number}`,
          },
        },
        quantity: 1,
      }],
      success_url: `${process.env.APP_URL}/invoices/${invoice.id}/paid`,
      cancel_url: `${process.env.APP_URL}/invoices/${invoice.id}`,
    });

    // Update invoice status and send payment link
    await db.invoices.update({
      where: { id: invoice.id },
      data: {
        signingStatus: "signed",
        signedAt: new Date(),
        paymentUrl: session.url,
      },
    });

    // Email the payment link to the client
    await sendPaymentEmail(invoice.clientEmail, session.url!);
  }

  return NextResponse.json({ received: true });
}