Tomorrow is the deadline if you plan on sending a parcel first class - to have it arrive by Christmas. After that - it's priority and express mail. Saturday is the last day to send a package with guarantee delivery by Christmas Day- sent by express mail.
There are a few options - if your looking to skip post office lines - the automated postal center is open to shippers all hours of the day - at the post office. You can ship with insurance - with delivery confirmation and even buy stamps or - for even more convenience - just jump online.