If you have a USPS package that you're expecting and need to sign for the USPS package or USPS mail and you're unavailable to sign for the mail then the USPS will usually make another attempt on the next working day to deliver the package or mail to you and get your signature.
You can also go on the USPS website and have the package rescheduled for a time and day you're gonna be available to sign for the package.
You may also be able to have the USPS hold the package at your local USPS where you can sign for the package and pick it up at the post office instead.