If you've got gifts for loved ones that you haven't sent yet, the U.S. Postal Service has some advice: Mail them...now.

Monday marked the busiest mailing day of the year in the U.S., according to the Post Office, with over 280 million letters, cards, packages and other items flowing through processing at more than 7,400 locations nationwide. That's nearly three times the daily average.

On Wednesday most of those items will be delivered, the peak of the Post Office's expected 20 billion holiday deliveries this year.

"The Postal Service is working around the clock to help make your holidays sparkle," Susan Plonkey, vice president for customer service, said in a press release.

Shipping company FedEx (FDX) also says the next few days encompass the most hectic time of the year. The company expected 9.8 million items to move Monday night.

All of this means that postal workers and other shipping services are making their appointed rounds, and then some, and that time is running out to get holiday goodies to friends and relatives.

In fact, the Postal Service now recommends using Priority Mail if you haven't already sent your packages. And after December 23, the Post Office says Express Mail is the way to go.

FedEx is also advising that at this late stage customers forgo ground shipping and use the company's expedited services to get gifts to their destination on time.

For more tips on holiday shipping:

Click here for Postal Service tips.

Click here for Federal Express advice.