As others have said, USPS is excellent. It's amazing what you can get into the flat rate boxes. Weight doesn't matter.
Because we live overseas, shopping can be difficult and mailing takes extra time. We got used to purchasing things online and having them mailed directly to our daughter. For USPS, just use the USMMA and box number. (As pointed out, these are not P.O. Boxes, but internal box numbers.) However, for places that ship via UPS or FedEx, use the 300 Steamboat Road address. The box will be delivered to the mail room and then directed to the correct person.
I will second the praise for the Mailroom staff. I've been known to include a box of chocolate for them when I send Belgian chocolate back for our regional mids at exam time.