Dive Brief:

• Despite other retailers making inroads in e-commerce sales via omnichannel moves, Amazon captured $23 billion more in U.S. e-commerce sales in 2015 than the previous year (up from $77 billion in 2014 to $100 billion) and accounted for some 60% of total U.S. online retail sales growth nationwide, according to Forrester Research’s latest U.S. online retail forecast, released Monday.

• Overall online retail sales in the U.S. will surpass $500 billion by 2020, compared to $373 billion in 2016, according to the report. Thanks in part to blurring of lines by omnichannel retailers, more than $1.5 trillion of total U.S. retail sales are now “web-impacted,” Forrester said.

• Mobile for now is a digital shopping channel that consumers are reserving for search rather than purchase: A huge 78% find their phone's screen too small or loading times too slow, according to research led by Forrester analyst Sucharita Mulpuru.

Dive Insight:

Experts say e-commerce is necessary for any retailer of any size, just because the industry’s center of gravity has shifted to omnichannel, what retail analyst Doug Stephens calls the "pervasive consumer belief" that goods should be available whenever and however they want to buy them.

Even though e-commerce transactions haven’t yet topped 8% of all sales, as a retailer “you better be able to do it either way,” Nick Egelanian, president of retail development consultants SiteWorks International, told Retail Dive. “If you have brick and mortar, you have to be able fulfill by mail. You have to be able to do it all the ways in which customer wants."

The Forrester report paints a picture of savvy retailers meeting that challenge, saying that efforts by “customer-obsessed” retailers that dedicate attention and resources to customer engagement are paying off, as are omnichannel efforts like endless aisle, ship-from-store and in-store pickup programs. The improving U.S. economy has also been key to the e-commerce sales boost.

Many obstacles remain, however. "As e-commerce has grown, variable costs have also increased, particularly in the heated race to acquire customers," Forrester states. "In particular, customers expect packages to arrive faster, so any economies of scale that may have come from volume discounts from carriers quickly vanish when retailers must pay to expedite orders."

Forrester notes that merchandising is key to competing, especially as Amazon is increasingly dedicating effort into offering its own private-label apparel brands, making it more difficult to compete on price in a commodified environment.

“The economics of retail are challenging and destined to remain low-margin for the foreseeable future,” according to the report. “While a category like apparel won’t be made obsolete by digital solutions, pricing pressure will continue to weigh on the sector, particularly as companies like Amazon invest in private-label apparel products. Companies that find new ways to monetize their businesses, through advertising or other adjacencies, will be better positioned than those that don’t.”