NEW YORK - The holiday hustle and bustle is reaching its peak for the U.S. Postal Service.

Postal workers say that Monday was the busiest mailing day of the year, with more than 600 million pieces of mail expected to be processed, which is nearly double its usual daily volume.

Several expedited shipping options are available to accommodate last-minute shoppers scrambling to send their holiday gifts and cards. The USPS says that priority mail flat rate boxes are available at your local post office, and as long as it's sent out by Dec. 21, it should make it in time for Christmas.

The busiest mailing day will be followed by the busiest delivery day of the year this Wednesday.