I did try that.

Paypal will not let you use AmEx for an airline ticket. I have tried that a couple of times, and that issue seems to go beyond UA. The other time I tried to use Paypal was because I did not have my AmEx physically with me and that was the card I wanted to use, so I tried it through Paypal. Paypal would not let me use Amex for a plane ticket so I just used a card I did have with me directly with the airline (US). AmEx is the only card that I have on Paypal and the only card I am going to have on Paypal, because Paypal's own chargeback bureaucracy is horrendous and I know AmEx will get the job done if I ever need a chargeback without having to go through the crap of dealing with Paypal first. I have had the Paypal chargeback experience and that is something I never never never want to have again. In fact that and another negative Paypal experience have left such a bad taste in my mouth for Paypal that I only use them as the last resort on anything.

I would much rather just use a different airline than put any other card on Paypal.