U.S. delivery firm FedEx Corp. posted a 7 percent drop in quarterly profit and cut its full-year forecast on Tuesday, after labor shortages crimped earnings, slowed packages and drove up costs ahead of the all-important holiday peak season. Shares in the Memphis, Tennessee-based company fell 4.6 percent to $240.50 in extended trading after FedEx said staffing problems resulted in a $450 million year-over-year increase in costs due to higher wage rates and overtime, increased spending on third-party transportation services and shipping hiccups. “The impact of constrained labor markets remains the biggest issue facing our business” and was a key driver for the first-quarter underperformance, FedEx Chief Operating Officer Raj Subramaniam said on a conference call with analysts. Most of the excess labor expense hit the company’s Ground network—which is now rerouting 600,000 packages a day, or 6.4 percent of the segment’s average daily volume during the quarter—to work around woes stemming from …