Consumers purchased more Halloween costumes on retail websites this year, while some online retailers struggled to keep a full supply of candy in stock, according to data from companies that track online sales.

Halloween is big business for retailers: The National Retail Federation, in its annual survey on the holiday, found that 179 million U.S. residents planned to partake in Halloween festivities, up from 171 million in 2016. The NRF projected that consumers would spend $9.1 billion this year, up from $8.4 billion in 2016.

Online sales of Halloween costumes increased 29% in October compared with a year ago, according to Slice Intelligence, which analyzes online sales based on the email receipts of a panel of 5 million consumers.

That represents an acceleration from the robust 22% growth in online sales of costumes in 2016 over 2015. Slice does not reveal specific sales numbers.

The average price of a costume in October was $26.76, which was slightly higher than the average price of $25.28 in September and $25.05 in August, according to Slice data. On average, a costume purchased online between November 2016 and July 2017 cost $22.75.

An inflatable Jurassic World T-Rex costume.

An inflatable Jurassic World T-Rex was the top-selling costume this year on Amazon.com Inc., No. 1 in the Internet Retailer 2017 Top 500, according to retail analytics firm One Click Retail, just as it was in 2016. The child-size version of this costume was the 10th best-selling costume on Amazon, down from No. 4 last year.

One Click Retail also finds that candy made by Mars Inc.—owner of such brands as Snickers, M&M’s and Skittles—took the four top spots in sales on Amazon.com this Halloween season.

Pricing analytics vendor Brand View monitored candy sales and online availability in October across the websites of several retailers. They included AmazonFresh (Amazon.com’s e-grocery service available in more than a dozen markets) in Manhattan, Philadelphia and Seattle; Safeway in McLean, Va., and Oakland; and Peapod LLC (No. 62) in Manhattan and Philadelphia. Significant differences emerged both in product availability and pricing strategy, Brand View found.

Candy supply is a critical factor to online sales during this peak season, especially the week before Halloween (Oct. 24-31), when many shoppers purchase candy, Elliott says.

“Retailers and manufacturers must ensure that a shopper is able to purchase their products,” Elliott says. “If a shopper can’t find the product, they are likely to switch to a rival and that sale, and potentially future sales, will be lost.”

E-retailer Peapod had 99.5% of its candy in stock for the entire week before Halloween in Manhattan, and 98% of its candy was available in Philadelphia. Safeway had similar high-availability percentages, with 88.7% available in McLean, Va., and 88% in Oakland.

AmazonFresh, however, had inconsistent candy supply, with only 67.1% available in Philadelphia and 62.5% in Manhattan. In Seattle, 89.1% of its candy was in stock for the entire season.

Candy manufacturers Mondelēz International, which owns brands such as Cadbury and Milka, and Mars had the most in-stock products across all of the analyzed merchants. Ferrara Candy Co.—which owns such candy brands as Trolli, LemonHead, RedHots and Brach’s—had the least consistent availability among the brands analyzed: Almost half of its products on the AmazonFresh Philadelphia website and 34.3% of goods on the AmazonFresh Manhattan website were unavailable for the week leading up to Halloween.

In terms of pricing, Safeway in Virginia cut its prices on the most candy products on Oct. 11, Oct. 23 and Oct. 25. In contrast, Amazon Fresh stepped up its price reductions in all three markets studied later in the season, on Oct. 18, Oct. 25, Oct. 27 and Oct. 28.