When you purchase a label and check out, your customers will receive a notice (at the email in your shipment file) that they have a package coming. We'll tell them their tracking number at that point, and will let them know that they won't see movement on that number until the package is actually on its way.
Once your packages are picked up by USPS, we'll notify them that the order is on its way and provide a projected delivery date (if USPS gives one to us).
If the package enters an "exception" status indicating a possible problem with delivery, we tell you (not your customer) so you have the opportunity to do any appropriate outreach with them to maintain satisfaction.
As the sender, you can always see up-to-date information about a package's transit status and tracking by going to "My Shipments" and clicking on the shipment batch to see details.