The biggest problem with USPS (where I live) is that they do not do front door deliveries.
Ergo, if the package does not fit in the mailbox they send it back to the central office.
They leave you a note, and you have to go there in person to pick it up.
An Amazon customer service person told me this is against standing Amazon policy.
Amazon is a front door delivery service. I have returned a couple of items because of this.
I do not actually return them, but if you do not pick it up USPS will 'return to sender'.