Paid for UPS shipping and the package was immediately sent to the US Post Office.

Did you immediately go back to the UPS store and tell them?

This may be a case of them doing it intentionally for even more profit, then if the customer notices and calls them out on it they say; "Oh, there must have been a mix up..."

Then you tell them to rectify it by refunding you the difference.

I chose UPS Saver one time. It said flexible delivery between 3 and 7 business days. It did NOT say what it was: delivering it to the nearest Post Office and then let them kick it down the road for 2 weeks. "Smart!"

From the all knowing internet…..

While USPS does not directly deliver packages labeled for UPS, they can sometimes deliver the final leg of a UPS shipment through a service called "UPS SurePost," where UPS transports the package to a local USPS facility for final delivery to the recipient's address; essentially, UPS uses USPS's network for the last mile delivery in certain situations. 

Key points about UPS SurePost: 

• Collaboration, not direct delivery:

  USPS does not deliver packages directly for UPS, but they partner through this specific service. 

• Cost-effective option:

  UPS SurePost is often used for lightweight or low-value packages as it can be cheaper than full UPS ground delivery due to utilizing USPS's final mile network. 

• How it works:

  UPS handles the initial transport, then hands off the package to USPS for the final delivery.