What to know about last-minute holiday shipping deadlines, avoiding fees

The holiday crunch at the post office is on.

The week before Christmas is traditionally the busiest week of the year for the carrier, which means procrastinators should expect to wait in line.

The agency's holiday mail tracker shows more than seven billion pieces of mail are already on the move nationwide.

And the post office says it is working 24-7 to keep everything on track.

To ensure packages make it in time for Christmas, the ground and first-class mail deadline is Wednesday, December 17.

The priority mail deadline is Thursday, December 18, with the priority mail express deadline Saturday, December 20.

Post Office locations nationwide will be closed on Christmas Day and New Year's Day.

Here are some additional options:

UPS 3-day

Packages being sent using three-day shipping with UPS must be sent by Dec. 19.

UPS also offers 2nd Day Air, which must ship by Dec. 22, and UPS Next Day Air, which has a Dec. 23 deadline.

FedEx Express Saver

The shipping deadline to ensure holiday packages arrive on time is Dec. 20.

Tips to save on holiday shipping

• As you're mailing gifts, double or triple check zip codes on the label to avoid any delays.
• Put a card inside the package with your return address just in case it gets damaged.
• Check with the store where you purchase a gift and ask about any free shipping deadlines.
• Ship packages directly to friend or family member when possible.
• Consider USPS Flat Rate boxes, which offers a range of envelope and box sizes, and won't charge customers more than the specified amount.

How to avoid potential tariff fees while holiday shopping

Anyone shopping gifts online right now may notice added fees related to tariffs, which can occur either at checkout -- separate from shipping and taxes -- or after your package is delivered.

According to the Mastercard Economics Institute, some popular holiday gifts getting hit especially hard with tariffs this season include clothes like leather gloves and sweaters, sports equipment and toys.

When buying goods from a retailer overseas, assume you might get charged a customs bill. To try to avoid this while shopping online, look for "import charges included" or "delivery duty paid" on the checkout page. If you don't see that, check the website's FAQ section or call the seller directly.

Keep in mind that once a package bought from overseas crosses the border, even if you try to return it, you could still be on the hook for paying any customs fees, so be sure to check before you buy.

ABC News contributed to this story.