LOS ANGELES (CBSLA.com) — The U.S. Postal Service Monday was preparing for the busiest shipping and mailing day of the year.

On Dec. 15, Americans will send more than 640 million cards, letters and packages, which is a 22 percent increase over the daily average of 523 million pieces of mail.

The USPS expects six million customers to visit Post Offices throughout the day.

Officials recommend customers use its website usps.com to avoid crowds and save fuel by buying and printing postage online and scheduling package pick-ups from their home or office.

Here are recommended mail-by dates:

Dec. 15: Standard Post
Dec. 17: Priority Express Military Service
Dec. 20: First-Class Mail
Dec. 20: Priority Mail
Dec. 23: Priority Mail Express