Plunging rates for chartering container vessels that carry sneakers, furniture and flat-screen TVs may signal a U.S. consumer slowdown and losses for shipping lines in what is traditionally their busiest time of the year. Fees for hiring vessels have fallen 9.3 percent since the end of April, according to the Howe Robinson Container Index, which tracks charter rates for a range of vessels. Last year, the index surged 56 percent in the period, as lines added ships on demand from U.S. and European retailers restocking for the back-to-school and holiday shopping periods. “The troubling part is that charter rates are...