"Suspicious activity" is also caused by logging in from a different IP, such as another computer or if you have to reset your router.

If SE isn't accepting your payment directly you can try buying Crysta through Ultimatepay. You can purchase specific amounts of Crysta so you don't get charged a few cents extra or whatever. Crysta purchases are available at the Mogstation under "Square Enix Account settings" and getting "Add Crysta".