Most of the time, when you pay a bill by mail, it goes to a PO Box. Is this an actual box at a post office somewhere, or is it actually a special delivery by the Postal Service directly to a company’s mailroom?
Most bill payments to large corporations go to what are called “lockboxes.” These are special PO Box numbers that are maintained at USPS central sorting plants. The USPS refers to this mail stream as “remittance mail.” This mail is not delivered to local post offices.
Lockboxes are assigned to banks. Banks offer customers lockbox services where they go and pick up the remittance mail from the processing plant several times a day and pick up the mail, open it, remove the contents, and process the checks. The banks make electronic images of the checks and remittance stubs and make those available to their clients. They can also do additional processing on the information for their clients.
The remittance mail is available for pick up as soon as it is processed. The processing plants work on remittance mail 363 days a year, 20 hours per day. Banks can call for the mail 24 hours a day, 363 days a year.
To watch a video on how this works, click here, then click on the “Watch Now” box below the picture. You will see a drop down, click on the “retail lockbox” label on the right side of the drop down.
But anyway, the simple answer is that the post office does provide a large box for large corps, and it can also forward it, as large corps run their own mail room anyway, and probably sends a lot too. so the post office visits often… drop some mail, pick up some mail … 
I’m guessing one of the two remaining days is Christmas. What’s the other one? New Year’s? July 4th?
That’s a great question. I can’t find an answer. They implemented the 363 day/20 hour processing schedule a couple of years ago when the started the processing center consolidation plan. Almost every document you can find mentions the 363 days, but nothing mentions what they are.