Sometimes when a package fails to be delivered, the package is currently at the post office that services your house. It will sit there until sometime tomorrow when all the mail goes out for the day.
What to Do
Bring your ID and a printout of the tracking number from the USPS website. That way, they can take the paper from you, enter the number in the computer, and then walk back with the printed-out you gave them to look for the package.
Also, tell the postal worker before he goes back to look at the package. It is a small white 9 x 6 bubble mailer with your name and address, and the sender is Summit Products. That will help them identify it from other packages on the return to sender or undeliverable mail shelf.
The postal worker will go back and check that shelf. If you are at the post office that services your house, it will likely be there.
I recommend arriving close to when the post office opens to get to the post office early before anything disturbs the placement of the packages. This will maximize your chances of getting the package from them.