Depends on the type of e-cert.

if it's one of the old United ones, which start with 016, then it's done as a discount on the fare and will NOT be included in the total amount as others have said.

If it's one of the newer "ETC" certs (or an ex-CO cert) which starts with 11 or 12, and has a PIN code then it will be shown as a payment. When you actually purchase it'll show 2 payment entries, one for the cert, one for any extra payment. If you ask for a receipt at the airport (kiosk) or from the website, it'll show the full amount of the "payment" including the cert value.