Now that USPS is including free delivery confirmation on first-class parcels/packages instead of the $0.19/ea (went into effect with their rate change this January), we've noticed a huge decrease in the number of inquiries/complaints and a big increase in customer satisfaction. In the Endicia software (is available for Macs), you can customize emails to automatically send when the postage is printed, whether it's from an automated/xml feed or not.
People have been trained that USPS is not as fast as FedEx/UPS because their tracking system was so frustratingly antiquated ("oh, that's just a CONFIRMATION number, not a TRACKING number"). USPS is quickly coming up to speed, and the third-party tools like Endicia/Dazzle are helping bring their new features to the forefront. Even if they caught on a little slowly, they see all the volume in parcels/online shipments and are fighting for their piece of the online pie.