Can I use a domain I already own instead of the address you gave me?

Yes. Settings → Your own web address, owner only. Your menu is then served on your own domain and everything — orders, WhatsApp, the lot — works exactly as before. Your Get Menu address keeps working forever and sends anyone arriving on it to your domain, so printed QR cards and links you have already sent stay alive. You buy the domain from a registrar yourself; we connect it by hand so nothing goes near a live site by accident.