The United States Postal Service is the independent postal service agency run by the federal government of the United States and it being a public facility is prone failures. The service which is commonly known as USPS is one of the most used postal services along with FedEx and UPS in the United States.
”Shipment received, package acceptance pending” is a normal message showing on the tracking but within the first 48 hours after postage. This is not the case always. Delays do happen. Mails might take some time to be processed at the sorting facility, especially during the festive season. But if the message is still showing beyond 2 days then you should contact the USPS customer care. They are the one who can tell you whats happening and exactly where your package is.
If it takes more than 15 days then your package might have been lost, broken, or stolen. There is also a possibility that the USPS employees have forgotten to ship it.
For better and faster service, you can try other companies like FedEx and UPS over the USPS in the future.