If you still haven’t sent your gifts to friends and family around the country, don’t worry—the carriers are ready to whisk your packages to their destinations in time for the holiday. But which courier should you use? 

The three big services—FedEx, United Parcel Service (UPS), and the U.S. Postal Service (USPS)—all deliver to nearly every address in the U.S., and they'll all get your parcels to more than 200 countries and territories. But there are some key differences.

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Price. If the cost of shipping that special gift is your main concern, the USPS is often the way to go, but it pays to check around. We compared shipping costs offered by FedEx, UPS, and the USPS between 30 pairs of cities around the country for a 5-pound medium-sized package. The USPS prices were best for next-day delivery in all cases and for third-day delivery in 88 percent of our city pairs. FedEx, however, was the least expensive courier for second-day service in every instance, but it was least expensive for third-day service in just 13 percent of deliveries. 

Packaging and delivery options. All of the carriers offer ways to reduce shipping costs, even for speedy delivery. At the USPS, the flat-rate box rate may be the best deal, but not always. In 31 percent of the city pairs we compared, Priority Mail was a dollar to a few dollars cheaper than the $13.45 medium flat-rate box for the Postal Service's "Two-Day" delivery option. (In either case, however, the post office estimated that those "Two-Day" packages would actually be delivered on the third day.)  

At UPS, the published rates for the "Next Day Air Saver" option were, unsurprisingly, less expensive than the “Next Day Air Early” option. The difference in price, though, was significant. Sending our medium-sized package from Seattle to Miami for "Saver" delivery by the end of the next day costs $92.49, for example. But it costs much more—$128—to deliver that same package by 8 a.m. the next day using the "Early" service.

At FedEx, you can lower your costs by choosing the "One Rate" option, which is typically cheaper than the courier's "Standard Rate" service. With One Rate, you select a standard box size, choose the kind of service you want (one-, two-, or three-day delivery), and pay the same fixed rate as long as the package is worth less than $100 and weighs less than 50 pounds. The Standard Rate, by contrast, takes the package weight and dimensions into account.

In our comparison, One Rate fees compared with Standard Rate ranged from 7 percent less on second-day service to 53 percent less for third-day delivery.  

Consider these other factors as well: 

Dependability. Although neither FedEx nor UPS would divulge its on-time performance data to us, both carriers delivered more than 98 to 99 percent of their packages on time year-round, with minimal declines during the hectic holiday season, according to ShipMatrix, a shipping software company that tracks millions of packages per year.

The USPS delivered 84 percent of packages on time from October through December 2015, according to its own quarterly performance filing. (ShipMatrix did not have sufficient data on USPS deliveries.) 

Delivery days. If you’re really leaving things to the last minute and you need a service that delivers on Christmas Day, your best bet among the three carriers is the USPS. Though FedEx and UPS deliver only 307 days of the year (no Sundays or holidays), the USPS’ Next Day Priority Mail Express and guaranteed delivery service operates 365 days per year to select ZIP codes. Be prepared to pay a $12.50 surcharge.

There’s one more thing to keep in mind. The USPS can work all kinds of magic to get your package to its destination on Christmas Day. But it’s up to you to get it to the post office. This year, you have until Dec. 23.