I've tried logging out and logging in but I still can't purchase the DLC with my card (got the same error while trying to buy other games as well ) :( maybe it's an itch.io issue?
Then I'm afraid you'll have to contact Itch themselves. We don't have any way to fix their site if it simply won't work. Sorry for the trouble!
It appears so. Paypal would be your best bet.
I've had issues on other sites where one payment method doesn't work (like paypal) but card does and vice-versa.
