United Parcel Service is straining to handle a surge in online sales that has resulted in more holiday volume than it had expected, causing a wave of disruptions that could spell trouble for the holiday season. On-time delivery rates for UPS ground packages based on their normal shipping transit times last week fell to 91%, according to an analysis of millions of packages by software developer ShipMatrix Inc. During the same week last year, the on-time rate was 97%, which is UPS’s usual average during nonpeak months.