Monday is the busiest shipping day of the year for the U.S. Post Office. Twenty billion pieces of mail are expected to be sent somewhere this holiday season; that includes cards, letters, and packages.

The longer you wait the more it will cost. To make sure your gifts arrive on time you have two deadlines.

Thursday it's going to be bumped up to priority and first class mail, and on Saturday you have to use express mail, which is more expensive.

If you cannot make to the Post Office during their business hours there is still an option. The Post Office has Automated Postal Centers, where you can use your credit or debit card, print out the postage, slap the label on the package and it's all ready to go.

These centers can be found at most Post Offices and are open 24-7.