For a customer without portal access, or for a one-off, ⋯ → Share payment link on an invoice (Chapter 5) creates a public page for that invoice alone. The customer sees your name and logo, the amount Due (or Paid in full), the invoice summary, your message and terms, Pay and Download invoice PDF. After paying they see Payment received; the payment is recorded against the invoice and a receipt is emailed. This payment link is invalid or has expired means the link was revoked or the invoice was voided.
Online payment — in the portal and on payment links — needs Settings → Integrations → Stripe to be connected. Without it the pages show the invoice and invite the customer to contact you for payment details.