Customers have been complaining to the company on social media, marking an inauspicious start to the crucial Christmas shopping season for Atlanta-based United Parcel Service Co. Along with rival FedEx Corp., UPS has benefited greatly in recent years from the growth in online shopping. While the company declined to put numbers on the delays, research firm ShipMatrix said last week that 89 percent of UPS Express packages were delivered on time. By comparison, FedEx's express service scored better than 94 percent. Spokesman Jess Bunn said FedEx was proud of its record and anticipated record volumes. The president of the Teamsters union, which represents UPS drivers, said the company wasn't staffed properly and has underestimated Cyber Monday purchases three straight years. In a letter to UPS CEO David Abney, James P. Hoffa also complained that the company plans to increase hours for drivers during the holidays up to 70 hours over an eight-day period.