I had the same problem. I ignored it though. Erm, they used to let you just go ahead and checkout, but I guess they took that feature out. I wanted to purchase another tb but I got the same message.
Edit: I did a little research on the site. It is a 1.95$ fee that allows you to spend extra fees. Whenever you get that message it means that you have reached your max in Paypal at the moment. Means, you have to buy the 4 digit code and it is sent to the credit card statement and/or sent in the mail.