Early in his days at UPS, the holidays didn't pose much of a challenge for Kelsey Cullum.

When he started, most packages were sent between businesses, and online shopping was still a novel idea. So working in the parcel service's Columbia and Florence warehouses in the 1990s and early 2000s, he noticed a bump in volume as Christmas approached, but it wasn't all that different from a regular day.

This year, Cullum faced a deluge of packages bound far-flung parts of Colleton and Hampton counties from the distribution center he runs near Hilton Head Island. Drivers looped through towns like Estill every day in the holiday season, he said, reaching 130 or so homes apiece on roads many of them had never even heard of.

"The majority of them, you see a lot of the Amazon on them, a lot of the QVC stuff on the side of the box," said Cullum, UPS's Hilton Head business manager. "In '95, you didn't see much of that."

And while that may not come as a surprise in an age of Amazon Prime and free shipping, the influx of Amazon boxes into rural corners of the state highlights just how commonplace and widespread online shopping has become.

The growth of rural shipping poses a mounting challenge for shippers who make more money when deliveries are bunched together, for online retailers who pick up the tab and for local shops who face competition they can't see.

Prime time

When Cullum got his start loading UPS trucks part-time, Amazon was just a startup out to sell books, not an e-commerce goliath. But now, the online retailer's Prime service has pushed far across the country.

By the end of 2015, almost a third of households in the U.S. - 31 percent - had a Prime subscription and access to its marquee lure: free, two-day shipping. Most of the service's growth that year didn't come from city centers, but from suburbs, towns and farther-flung homes, according to the market research firm Kantar Retail.

And so these days, rural consumers - people who have to drive at least 10 miles to do their shopping - have Prime accounts at rates close to the national average, and they browse and buy online at much the same rates as everyone else, Kantar found.

That makes sense, because the promise of e-commerce - of buying just about anything and have it shipped to your door - is perhaps most potent outside of cities, in places where the store isn't as close by. Cullum figures many of the packages moving through his center are headed to people who don't want to drive to Aiken or Walterboro.

"It's an hour either way, and that's going to be an all-day event," Cullum said. "A lot of the things, I would say, are probably things you or I could get at Wal-Mart, but it takes them hours."

'Last mile'

That convenience comes at a cost: Every drive a shopper doesn't have to make, a delivery truck does. And in the shipping business, density is the key to profits.

Moving a box the "last mile" of its trip - from a distribution center to a front porch - is the costliest part of shipping, but it's easier to manage in a populous city center. Handling an extra package in Charleston, for example, doesn't add much to UPS's costs, maybe just another stop on a King Street route. Making another delivery in rural Dorchester County, though, could mean a few extra miles of driving, which can add up as home delivery grows even more.

Oftentimes, those extra costs are passed on to retailers. UPS and FedEx both charge surcharges for rural ZIP codes - a few extra dollars per box - and e-commerce companies often eat those costs.

In the age of Amazon Prime, free shipping has morphed from a luxury or a bonus to an expectation. Almost nine in 10 consumers say it makes them more likely to shop online, according to a survey commissioned by the marketing firm Walker Sands, so companies have shown a willingness to carry the extra cost.

That's also changing how people shop on the Internet, according to Blue Acorn, a Charleston-based tech company that builds e-commerce websites. They're spending more online, mostly because they're placing smaller orders more often.

"We infer that this is, in part, due to the standardization of free shipping," data analyst Samantha Previte said in an email.

To keep up with those trends, parcel services are experimenting with new ways to cut delivery costs, particularly in dense areas: Package lockers are becoming more common nationwide, autonomous vehicles could eventually carry boxes to them and services like Uber are angling to unseat companies like FedEx and UPS for the future of last-mile delivery.

Rural areas, however, are trickier. The consulting firm McKinsey predicts the challenge of shipping density in rural areas will eventually be solved by package lockers and drones. But lockers aren't likely to be as convenient as they are in cities, and while rural areas limit the safety concerns raised by airborne delivery, drones are still years away. A network of unmanned aircraft would be enormously complicated to launch and manage, and they're not legal under the current federal regulations.

Tech threat

While the technology and logistics sectors work to make rural deliveries cheaper, it won't do much to ease the threat of e-commerce to small-town businesses.

Big-box stores have long been blamed for the demise of Main Streets and local businesses, but shopkeepers now see web-based competition as a bigger threat.

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In one recent survey, 70 percent of independent retailers said competition from the Internet was one of their main concerns - almost twice the number that listed big national chains as a top challenge.

In Moncks Corner, Manuel Cohen said he doesn't mind the competition he faces from the Wal-Mart down the road. He figures his shop, Barron's Department Store, stands out with higher-end products and a promise of better service, and if nothing else, having more folks drive into town only helps business.

But he says e-commerce has probably dented his sales, even as customers from farther-out towns like Andrews and Holly Hill come to his store. They still come in to try on a sport coat and see how it fits, but he guesses that some of them will be tempted to buy one from their couches. 

The Institute for Local Self-Reliance, the think tank that conducted the survey of retailers, says that temptation has sweeping impacts: It threatens retail jobs, towns' tax revenues and the liveliness of their commercial districts.

Blurred lines

Most challenging of all, the group says, is that online shopping is distributed and hard to track. Unlike a new store across town, it's hard to draw a clear cause-and-effect between a store's struggles and e-commerce's rise.

"It leaves small signs scattered around cities - a white delivery truck with its logo on one side, or its boxes piled outside on recycling day - but when the toy store closes down, it’s hard to know the degree to which its customers had started shopping at Amazon," the group said in a November report.

That may be the case in St. George, where Charles Weeks says 2016 was probably the worst year he'd seen at Weeks' Department Store, a mainstay of the Dorchester County seat since 1944.

Maybe it's that his customers are more willing to drive somewhere bigger, says Weeks, who's lived in the town for nearly all of his 77 years. Maybe his inventory isn't quite right. Or maybe it's that there's just not enough else to attract people downtown.

Maybe it's online shopping, too. He's heard delivery drivers say they're carrying a lot of packages in town, but he's not sure how much of an impact that's having on his bottom line.

But whatever it is, it's not just St. George. The outlet mall in Santee closed a few years back, he points out, and Ferse’s 5 & 10 in Orangeburg closed last year after more than a century. In any case, he says he'd like to see more stores in town angling for business.

"The more you've got pulling, the better off you are," Weeks said. "There's just not a whole lot pulling downtown."

Reach Thad Moore at 843-937-5703 or on Twitter @thadmoore.