In this case, "Package transferred to post office" means your package has been transferred to USPS.
This happens when a package shipped via UPS is then transferred to USPS before it reaches its final destination. USPS is a faster and more reliable postal service for shipping small packages, and UPS will often send their small packages to USPS for final delivery. This does not mean you need to go to your local post office to pick it up. The package is still on its way to you.