Phase 5: Seeded 6 email templates (billing_invoice_created, billing_payment_success, billing_payment_failed, billing_renewal_reminder, billing_past_due_reminder, billing_account_suspended). Added 8 notification methods to BillingService (sendBillingEmail, notifyInvoiceCreated, notifyPaymentSuccess, notifyPaymentFailed, notifyPastDue, notifyUpcomingRenewal, notifyAccountSuspended). Added processDunning() with escalation: reminders at 1/7/14 days, auto-suspend at 30 days (sets domain_names_sites.online_status=0). Wired notifications into generateInvoice and processAutoPayment. Added dunning to cron. Phase 6: Created InvoicePdfService.php using DomPDF. Professional invoice PDF with grouped line items, totals, styled layout. Added downloadInvoicePdf controller action   PDF buttons on invoices list and invoice detail. Tested: INV-2026-03-0002.pdf generated (3,643 bytes).