The retail calendar has accumulated so many shopping holidays that they now bump into one another like commuters on a rush-hour subway platform. October's Prime Day — the autumnal younger sibling of the June original — has become the latest occasion for advertisers to open their wallets, though this year they are doing so with slightly less abandon than in seasons past.

According to media buyers at several agencies, client spending on Amazon advertising in the weeks before the fall event rose between 15 and 40 percent compared with last year, a healthy increase that nonetheless represents a deceleration from the 40-to-50-percent leaps of earlier vintage. Economic uncertainty, it seems, has a way of encouraging restraint even among brands that have come to view Amazon as indispensable.

"From an endemic standpoint, we're looking at 30 percent growth year on year," said Darren D'Altorio, senior vice president of paid media at Wpromote. "From a non-endemic standpoint, it's 40 percent year on year."

The more interesting development may be where those dollars are going. Advertisers are increasingly directing their budgets toward Amazon's demand-side platform and its video inventory — Prime Video, connected television, even Twitch — rather than the sponsored product listings that have long been the platform's bread and butter. Katie Davis, director of retail marketing at Wpromote, noted that the DSP "continues to be viewed more as a brand building lever for media dollars than a retail checkout driver."

(Translation: advertisers have discovered that they can use Amazon's considerable knowledge of what people buy to reach them before they are actively shopping, which is the sort of insight that tends to loosen purse strings in planning meetings.)

Non-endemic brands — those that do not actually sell products through Amazon — have found Prime Video to be a particularly inviting entry point, according to Joe O'Connor, senior director of innovation and growth for Amazon at Tinuiti. The Federal Trade Commission's continuing investigation into Amazon's ad auction practices appears not to have dampened enthusiasm. "At the end of the day, what our clients care about is performance," Mr. O'Connor said.

Some buyers cautioned that the October event, shortened this year to two days from four, may prompt reconsideration, particularly with Black Friday lurking just weeks away. But for now, the spending continues, with Mr. D'Altorio noting that Amazon has become easier to justify to the finance department — which, on Madison Avenue, is often the only audience that truly matters.

Original story published in Digiday: "Fall Prime Day catalyses Amazon video and DSP spend growth", by Sam Bradley, Krystal Scanlon