No REAL ID yet? You can still fly, but it may cost $45 without another form of accepted ID

That little star on your U.S. driver's license is about to save you $45.

Beginning Sunday, air travelers in the U.S. without a REAL ID or another acceptable form of identification, such as a passport, are subject to a new fee.

It isn't a penalty or a fine — it's payment for non-compliant travelers to use the Transportation Security Administration's new alternate identity verification option called ConfirmID. But the process takes extra time, and paying the fee doesn't guarantee you'll make your flight.

The Department of Homeland Security says most U.S. travelers are already compliant and that the fee is meant to encourage the rest — those without a star-marked REAL ID — to obtain one.

Still, the new fee may catch some passengers off guard, so here's a breakdown:

What is a REAL ID?

It is a federally compliant state-issued license or identification card that meets enhanced requirements mandated in the aftermath of the Sept. 11, 2001, terrorist attacks.

The updated ID is indicated by a white star in a yellow circle in most U.S. states. It has been required since May, but travelers without it — and without another TSA-accepted ID — were still allowed through security with extra screening and a warning. That changes Sunday.

Who has to pay?

Travelers 18 and older flying domestically without proper identification on them will have to pay $45 to verify their identity at the airport through the ConfirmID process. If approved, the verification covers a 10-day travel period.

Paying the fee, however, does not guarantee access to U.S. air travel, and passengers whose identities cannot be verified may be turned away at airport security.

''This fee ensures that non-compliant travelers, not taxpayers, cover the cost of processing travelers without acceptable IDs,'' said Adam Stahl, acting TSA deputy administrator.

The service fee is paid online at tsa.gov/ConfirmID. Travelers will have to enter their legal name and the start date of their travel. Accepted payment methods include debit and credit cards, Venmo and PayPal.

Someone other than the traveler can make the payment, according to TSA, but the traveler's information must be entered correctly.

Passengers will then receive an emailed payment receipt from pay.gov.

At the airport checkpoint, show a government-issued ID and a digital or printed copy of the receipt to begin the verification process.

TSA recommends that travelers pay the fee before arriving at the airport to save on time, as the verification process alone could take up to 30 minutes.

TSA accepts digital IDs through platforms such as Apple Wallet and Google Wallet at more than 250 airport checkpoints in the U.S.

The agency has a full list of acceptable IDs on its website. They include:

— Passport or passport card

— Permanent resident cards

— Trusted traveler cards, such as Global Entry or NEXUS