Is Apple Pay Later usable for airline tickets?

Yes, according to the "Related Article:"

Apple Pay Later lets you split a purchase into four equal payments over six weeks with no interest or fees.1 When you use Apple Pay Later to make a purchase, we pay the merchant, and you repay us for your purchase over time. There's no impact to your credit score when you apply for an Apple Pay Later loan, and you'll know in seconds if you're approved.2

Apple Pay Later is available for eligible purchases between $75 and $1,000 made on iPhone or iPad devices at most websites and apps that accept Apple Pay.3

What you need

• Be 18 years of age or older.4
• Be a U.S. citizen or a lawful resident with a valid, physical U.S. address that's not a P.O. Box.5
• Set up Apple Pay with an eligible debit card on your device. You can only make Apple Pay Later down payments using a debit card.
• Set up two-factor authentication for your Apple ID and update to the latest version of iOS or iPadOS.
• You might need to verify your identity with a text message, Driver License, or State-issued photo ID.6

How to set up and apply 

• Every purchase made using Apple Pay Later is a separate loan and requires a new application. You can set up Apple Pay Later in Wallet or by applying at checkout. The first time that you apply, you'll be asked for basic information to process your application. After that, the information that you provided is used for future applications. You can update your information with us at any time.

You are welcome.