This usually indicates the carrier has not taken possession of the package. We continuously ship orders, even after the carrier arrives. All orders processed after the carrier arrives leaves with them the next business day.
When you receive your tracking email and your shipment notification, it is only an indicator that your order was confirmed, picked, packaged and the shipping label created. We have no control over what times the USPS arrives and it is random every day. UPS on the other hand consistently arrives around 4:00pm EST to 5:00pm EST.