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Walmart online shoppers are finding it more difficult to order certain items — heavier products like detergent and pet food and bulkier multi-item orders that are expensive to ship — The Wall Street Journal reports.

E-commerce fulfillment is the bane of any online retailer, which is why solving the last mile has emerged as e-commerce’s Holy Grail.

Shipping and fulfillment costs are bedeviling all players, including Amazon, which has arguably set consumer expectations for fast and free shipping. The e-commerce giant’s shipping costs rose 31% year over year in its most recent quarter, after rising more than 30% (and usually more than a third) in the last five quarters, according to its second quarter earnings report.

The company for half a century has staked its own success on fiercely efficient retail distribution, playing hardball with suppliers and honing its supply chain to maximize profits. But it’s a system based on legacy retail, where the customer supplies free labor to cover the last mile: taking items off shelves, bringing them to checkout, loading them into the vehicle and getting it all home.

Walmart has struggled to bring the same level of efficiency to its online sales, though it continues to experiment. Last month, the company revamped a pilot program, in which store employees deliver online orders. And, the company says it will have expanded its prominent in-store pickup towers to 700 stores by year’s end..