Build an Invoicing and Payment Reminder Tool

Tables: invoices (issuer_id, client_name, client_email, items[], subtotal, tax, total, currency, due_date, status, paystack_ref, paid_at), reminder_log (invoice_id, reminder_type, sent_at).

async function createInvoice(issuerId, clientEmail, clientName, items, dueDate) {
  var subtotal = items.reduce(function(sum, item) { return sum + (item.qty * item.unit_price); }, 0);
  var tax = Math.floor(subtotal * 0.075); // 7.5% VAT (Nigeria)
  var total = subtotal + tax;
  var reference = 'INV-' + Date.now();

  var invoice = await db.invoices.create({
    issuer_id: issuerId,
    client_name: clientName,
    client_email: clientEmail,
    items,
    subtotal,
    tax,
    total,
    due_date: dueDate,
    status: 'sent',
    paystack_ref: reference,
  });

  // Generate Paystack payment link
  var res = await fetch('https://api.paystack.co/transaction/initialize', {
    method: 'POST',
    headers: { Authorization: 'Bearer ' + process.env.PAYSTACK_SECRET_KEY, 'Content-Type': 'application/json' },
    body: JSON.stringify({
      email: clientEmail,
      amount: total,
      currency: 'NGN',
      reference,
      metadata: { invoice_id: invoice.id, issuer_id: issuerId },
    }),
  });
  var paymentUrl = (await res.json()).data.authorization_url;

  await sendInvoiceEmail(clientEmail, clientName, invoice, paymentUrl);
  return { invoice, paymentUrl };
}

// Webhook: mark invoice paid
if (event.event === 'charge.success') {
  var meta = event.data.metadata;
  if (meta.invoice_id) {
    await db.invoices.update(meta.invoice_id, { status: 'paid', paid_at: new Date() });
    await generateAndSendReceipt(meta.invoice_id, event.data.reference);
  }
}

// Cron: runs daily to detect overdue invoices and send reminders
async function processOverdueInvoices() {
  var today = new Date();
  var pendingInvoices = await db.invoices.findAll({ status: 'sent' });

  for (var invoice of pendingInvoices) {
    var daysOverdue = Math.floor((today - new Date(invoice.due_date)) / (1000 * 60 * 60 * 24));
    if (daysOverdue <= 0) continue; // not yet overdue

    var remindersAlreadySent = await db.reminderLog.findByInvoice(invoice.id);
    var sentTypes = remindersAlreadySent.map(function(r) { return r.reminder_type; });

    var toSend = null;
    if (daysOverdue >= 3 && !sentTypes.includes('3day'))  toSend = '3day';
    if (daysOverdue >= 7 && !sentTypes.includes('7day'))  toSend = '7day';
    if (daysOverdue >= 14 && !sentTypes.includes('14day')) toSend = '14day';

    if (toSend) {
      // Re-initialize payment link (same reference re-usable until paid)
      var paymentUrl = await getPaymentUrl(invoice.paystack_ref, invoice.client_email, invoice.total);
      await sendReminderEmail(invoice.client_email, invoice.client_name, invoice, paymentUrl, toSend);
      await db.reminderLog.create({ invoice_id: invoice.id, reminder_type: toSend, sent_at: new Date() });

      if (toSend === '14day') {
        await db.invoices.update(invoice.id, { status: 'overdue' });
        await notifyIssuer(invoice.issuer_id, 'Invoice #' + invoice.id + ' is 14 days overdue.');
      }
    }
  }
}

// Mark invoice as void if client and issuer agree it should not be paid
async function voidInvoice(invoiceId) {
  var invoice = await db.invoices.findById(invoiceId);
  if (invoice.status === 'paid') throw new Error('Cannot void a paid invoice');
  await db.invoices.update(invoiceId, { status: 'void' });
}

See build a freelance escrow style platform for a related tool that holds payment until work is approved.

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