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Can I see a screenshot showing how the original symptom looks like from your side?
As no further input was gathered against my last clarifying question, I now provide a general answer based on the given description and common situations.
As per your situation (running the latest macOS and the issue with flight booking only), it was highly likely caused by an issue of the dirty date of the booking website. You might have been visiting the same website one than one day. The solution is to delete all web history and website data of your web browser, then sign in the booking website and make a booking then make a payment.
You may also try using a different browser if the problem persists with a web browser.