I frequently get a payment error when trying to book a ticket on Southwest. This never happens with other airlines (not even BA which can be tricky). I enter the billing details exactly as they are on my card and statement. It even happens sometimes when I choose a card that I've previously used and saved to my SW account. There is not a rhyme or reason that I can figure out, but it's not uncommon for me to have to enter payment details 3 or 4 times (I'll enter the card I want to use, it'll get kicked back, I'll enter it again thinking I made an error. Then I'll try a different card. Then finally a 3rd card will work).

What's going on? I can't be the only one.