“Citigroup Inc. plans to let credit-card customers finance big purchases on Amazon.com over longer periods of time — a deal that may spur sales at the e-commerce giant while boosting the bank’s interest-bearing balances.    Cardholders who’ve shopped on Amazon in the past 12 months can choose to pay off transactions of at least $100 on a longer schedule and at a lower annual percentage rate, the lender said in an emailed statement.”