Christmas is less than one week away. The USPS is expecting to deliver more than 800 million pieces of mail between Monday and Tuesday, a 33-percent increase over regular mailing days.
To help people avoid the rush, some post offices were open on Sunday.
Customers can avoid the long lines by doing their shipping online.
"Over 47 million people will visit our website during this holiday season. You can buy your stamps there. You can ship packages, download postage and schedule a free package pickup," Rich Maher of the USPS.
The U.S. Postal Service expects to deliver an estimated 16.5 billion cards and packages this holiday season.
Suggested mailing dates to ensure delivery by Dec. 25:
- First Class Mail - Tuesday, Dec. 20
- Priority Mail - Wednesday, Dec. 21
- Express Mail - Thursday, Dec. 22
For the deadlines for particular areas or different types of mail, go to www.usps.com/shippingdates.
For holiday tips and information, go to www.usps.com/holiday.
To find a USPS branch in your area, go to www.usps.com, click "Find Locations" and enter your zip code.