Costco is now offering delivery for its popular sheet cakes and party platters, adding a new convenience option for customers ordering customizable items.

Instacart, Costco’s primary delivery partner, announced Wednesday that shoppers can now order bakery and deli items through its marketplace and SameDay Costco service and customize them ahead of delivery.

The newly available items include Costco’s custom sheet cakes and 10-inch round cakes. Customers can also order sandwich platters, shrimp trays and fruit platters. The half-sheet cake in particular has long been a fixture of graduations, office parties and other large gatherings, and the made-to-order items rank among the retailer's most recognizable bakery offerings.

The change represents a significant expansion of Costco’s digital offerings for the fan-favorite products, which were previously unavailable for online ordering until February, when Costco ended a decades-old system that required customers to visit a warehouse and complete paper order forms. The customizable items had been a holdout as grocers moved more of their catalogs online, and competitors including Walmart and Sam's Club already offered online ordering for custom cakes.

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"For nearly a decade, we've worked closely with Costco to expand the ways their members can shop online, and this is another exciting step in that partnership," Ryan Hamburger, chief commercial officer at Instacart, said in a statement.

"We're proud to bring Costco's beloved made-to-order bakery and deli to sameday.costco.com and the Instacart marketplace, making delivery available for the first time and giving members even more ways to order for life's biggest celebrations."

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According to Instacart, shoppers can choose the size, shape and flavor of their cake from a catalog of available options.

Customers can also select from a preset list of designs and add a message of up to 30 characters in a color of their choice.

Wednesday’s announcement builds on Instacart’s long-running partnership with Costco, which began in 2017 with the launch of same-day delivery. Instacart, whose retail partners also include grocers such as Aldi, Publix and Sam's Club, has broadened online grocery beyond shelf-stable staples into fresh, prepared and now made-to-order foods.

Since then, the companies have expanded their partnership to include Costco’s same-day e-commerce sites.

Costco did not immediately respond to FOX Business’ request for comment.