USPS Priority mail is not a guaranteed and does not come with a money back guarantee.
Here is a link to the USPS priority mail website.
Here is the disclaimer at the bottom of the page.
Disclaimers In most cases, the expected delivery date printed on your receipt or provided at checkout will reflect a delivery time of 1, 2, or 3 business days and is based on origin, destination, and drop-off time. The expected delivery date does not come with a money-back guarantee