Amazon has been using its boxes to advertise its own products and services for years, but now the e-commerce giant is realizing that there might be some money to be made by shipping customers’ packages in cartons branded by paying advertisers.

AdWeek notes that Amazon recently began delivering some parcels in bright yellow boxes advertising the upcoming animated movie Minions.

Analysts say it’s a smart move on Amazon’s part as Walmart preps to launch its competing ShippingPass subscription service.

Most shipping boxes just go straight into the recycling pile, so many consumers won’t be bothered if there’s an ad on the outside, especially if the revenue helps to keep shipping costs down or free. The e-tailer recently announced that a number of smaller items will now ship at no cost, even to non-Prime subscribers.

“We know consumers are obsessed with free shipping and that it’s become nearly an expectation of online businesses,” one analyst tells AdWeek. “But it’s obvious Amazon can’t sustain the free-shipping model that’s kept consumers so devotedly in their pocket. So this is definitely an experiment to mitigate the loss they’re taking on that front.”

For the brands, it could be an improvement over outdated modes of advertising like direct mail, as one is more likely to notice a brightly colored shipping box than they would a postcard that gets lumped in with the rest of their junk mail.

Some experts do note that it’s probably best for both Amazon and advertisers to use this sort of campaign sparingly.

“Less is more,” explains a brand consultant. “If their boxes start to look like NASCAR race cars, they will shoot themselves in the foot. No advertisers will find value in that, or it will polarize consumers, who might then learn to ignore the box.”

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