X2. That's how Barclays makes money. People miss the last payment and they charge interest on the whole thing all the way back to day 1. They know a certain percentage won't pay it all off on time, and they use that history in determining how much they charge Apple for accepting the card.
Also, be sure the last payment is made by one year from the PURCHASE date, which may not be the same as the credit card statement date in month 12.
Better yet, set up an automatic payment plan on Barclays' site to be sure that the payment hits in time.
In general, the only redeeming feature of the BarclayCard is the deferred interest period. Other cards have better features otherwise. I guess it is no surprise that I just got an e-mail from them offering me another 12-month deferred interest period if I spend $1000 or more at the Apple Store in July. I haven't used the card in a year, and just paid for my 2012 Air using my miles card, so I won't be taking them up on the offer.
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Card companies used to have a derogatory term for people like me, who put everything on our cards and never carry a balance. That said, doing that for 18 years gave me a credit score in the 800s. I agree that it is better to use cards than leave them around. However, don't go on a cancellation spree since your total debt as a percentage of available credit also factors into your score. Get rid of cards gradually, as your limit on other cards goes up.