Good news for last-minute online shoppers: Many retailers have pushed back their deadlines for ordering gifts to be delivered before Christmas. But experts say shoppers shouldn't test the new limits.

Free Shipping Day -- when big retailers traditionally offer a last shot at free standard shipping in time for delivery by Christmas Eve -- falls a day earlier this year, on Dec. 16. But while nearly half of retailers had their order cutoff last year on or before that day, just 28% will do so this year, according to the National Retail Federation. Of the rest, 22% promise Christmas Eve delivery for standard-shipping orders placed by Dec. 19, and another 13% for orders as late as Dec. 21.

Shoppers get extra time this year because Christmas falls on a Sunday, allowing for Saturday delivery, says Vicki Cantrell, executive director of the NRF's Shop.org. Retailers have also become more efficient at processing online orders faster, she adds.

That doesn't mean consumers should wait to place their orders, retail experts say -- especially if the gift is a must-have for under the tree and can't easily be found in stores. Forrester Research expects a 15% jump in online sales this holiday season, versus the NRF's expected 2.8% jump in retail sales overall. That translates to a surge in online orders that not every retailer is well prepared for, says R.J. Hottovy, an equity analyst for Morningstar. "There's been some talk of stress on the system," he says. Smaller merchants and manufacturers selling goods directly may not have the staff to process the additional orders quickly, he says.

Here's how to make sure your gifts arrive before Santa does:

Factor in processing time

Two-day shipping doesn't necessarily mean that today's order will show up in two days, says Jordy Leiser, co-founder of customer service rating site StellaService. Many stores allow themselves another day or two to process orders, so be sure to check the estimated delivery time frame in your online cart before confirming a purchase, he says. Not every store has 24/7 operations, either, so someone placing an order after 5 p.m. on a weekday or any time on a weekend may not see their order status change until the next business day.

Look for a deal

Free shipping is a popular promotion this year, and Free Shipping Day will have plenty of offers too (hence the name). Some retailers will offer free shipping automatically on orders of a certain amount, others will do so with special coupon codes, says Luke Knowles, founder of deal site FreeShipping.org, which organizes Free Shipping Day. Even procrastinators may see discounts on expedited shipping. By the NRF's data, some 5% of merchants will offer such a deal for orders placed as late as Dec. 23.

Read beyond the ads

Take a promise of delivery by Christmas Eve with a grain of salt, say experts: Such guarantees often cover only delays on the store's part, and exclude delays related to weather or problems with the shipper. Deadlines and details also vary widely. Best Buy promises to credit shipping charges and send an e-gift card of $10 to $20; Amazon promises to refund shipping charges. Ads touting later deadlines may also gloss over just how much more you'd have to pay to get it there, Hottovy says. "The closer you get to the actual event, the more you're going to pay to expedite it," he says. Read all the details before counting on a guarantee, or paying extra for faster service.

Re-assess store pickup

Some big retailers, including Bed Bath & Beyond, Sears and Best Buy, let shoppers reserve items online for pick up in store, Leiser says. But there's a big difference between such services, which rely on what's already in stock at stores, and ship-to-store services, which send items from the warehouse to a local store. The latter -- available at stores including Wal-Mart, Payless and RadioShack -- isn't always faster than standard shipping, which means it may not make it there by Christmas Eve. Check estimates of when an order will be available for in-store pickup, and if it's being shipped to the store, how many business days it's expected to take, he says.

Error-proof delivery

"It's worth the five or 10 minutes to check the store's shipping policy," says Knowles. (It's usually available as a link amid the other terms and conditions, at the bottom of the store's web page.) Having an order sent to a P.O. box may add a day to delivery time or cause the package to be sent back, for example. Electronics, wine and other items may also require a signature or can't be left unattended, which could require several delivery attempts, he says.