The days when lumbering, hulking trucks ruled the world of package delivery might someday may come to an end, as the roads make way for smaller, lighter delivery vehicles. Amazon tested package delivery using licensed cabs in San Francisco and Los Angeles this fall, as it looks into using the vehicles to bring customers their goods.

Amazon paired up with mobile app Flywheel, reports the Wall Street Journal, citing sources familiar with the matter, as part of Amazon’s latest experiment to get packages out faster than brick-and-mortar competitors. This way, it doesn’t have to rely on traditional carriers like UPS, FedEx and the USPS, which all of which were left scrambling with shipping delays last holiday season.

It sounds like the test was carried out on the downlow — Flywheel cars made deliveries usually in the early morning when fewer people needed rides and customers and competitors weren’t as likely to notice what was going on, insiders told the WSJ.

Amazon would order up cab through the app, have them come to mini-distribution centers and pick up as many as 10 packages going to the same ZIP code. It paid about $5 per package for delivery within an hour, the sources said.

Meanwhile, brick-and-mortar stores are trying to one-up Amazon by turning stores into mini-warehouses that can provide same-day and even one-hour delivery, as well as the option to pick up orders that were placed online.

“Amazon really created this market for faster and faster delivery, and if you’re a retailer, you’ve got to do something to compete against that,” Greg Bettinelli, a partner at Upfront Ventures, which is an investor in same-day delivery startup Deliv told the WSJ. “But that also means Amazon can’t stand still, they’ve got to keep innovating.”

Amazon Is Testing Taxis for Deliveries [Wall Street Journal]

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