Strength in numbers: Sincerely launches iPhone API with 12 partners

Sincerely, the San Francisco startup best known for the Postagram app, took a hit earlier this week when consumer electronics giant Apple (s AAPL) announced its new Cards app for designing, printing and sending personalized photo greeting cards. But Sincerely, which aims to make it easy to get printed copies of photos taken on mobile devices, is fighting back in a classic way: by finding strength in numbers.

On Thursday, Sincerely announced the launch of its Sincerely Ship iOS Library, a platform that¬†lets any iPhone developer add the company’s “print and ship” functionality into its app — meaning any iOS app can allow users to turn a photo into a printed postcard and mail it anywhere in the world.

As part of the launch, Sincerely has teamed up with 12 corporate partners who have already created iOS apps on the Sincerely Ship platform. These partners include travel guide company Lonely Planet, which on Thursday is debuting “Lonely Planet Postcards,” and photo-sharing startups Path and Waddle.

Once a developer builds the Sincerely Ship’s “Print, Ship” capability into an app, Sincerely handles all billing, fulfillment and customer support. A revenue share is paid to partners based on the amount and price of products purchased —¬†generally 70 percent of all revenue above 99 cents per transaction.

It’s a smart move for Sincerely, and it couldn’t come at a better time. Now that it counts Apple as one of its competitors, banding together with an army of app developers may be the best way for the company to keep an edge.