Paying with CC: typing the 16 digits of the CC, expiration date, 'CVC', and then having your phone nearby for confirmation. Happens on a myriad of forms from various providers that may or may not save or log the CC number. Many times the text message doesn't arrive on time on your phone, which makes the transaction fail. Or you have reached your credit card 'limit' and the transaction also fails (without you knowing why).
Paying with Paypal: username and password (or password only if the username is saved), done. Happens only on the Paypal website. Funds are withdrawn immediately from your bank account if you want. Never fails.