Open up your mind and say, ahh...

Social gifting startup Sincerely, makers of Postagram and PopBooth, is bringing its popular mobile postcard making, printing and shipping features to an iPhone app you already love.

Sincerely is launching Ship, an iOS Library and SDK, as a plug-and-play software kit that developers can use to turn their iPhone photo apps into mobile printing studios.

“The printed photo is the world’s simplest but most ubiquitously appreciated gift,” Sincerely CEO and cofounder Matt Brezina says. “This extends our platform to a much broader user base. Right now, we have two apps in the App Store, tomorrow will have 14 … a couple months from now will have 100 apps in the App Store that are all exposing the Sincerely experience.”

More than a dozen launch partners will feature Sincerely’s print and ship technology in their iPhone apps, starting Thursday. Partners include Lonely Planet, Path, Pic Collage, Waddle, Quiption, Picsicle, Filtermania, Ship Mate, Interlacer, Pregnancy Progress, I’m Awesome and Color Effects.

At launch, Sincerely will continue to hand select app partners and evaluate interested parties on a case-by-case basis, Brezina says. The startup has already fielded inquiries from more than 200 developers, he adds.

App makers will earn 70% of revenue earned from the sale of each postcard above $0.99. Sincerely prices its postcards at $0.99 a pop, a fee that includes printing and shipping costs to anywhere in the world. App makers can keep the $0.99 price point in tact or price postcards higher to make a profit on sales through their apps.

Coincidentally, Sincerely’s SDK release comes just two days after Apple announced that it would enter the greeting card business with Cards, an iPhone application for creating, printing and sending greeting cards.

Brezina sees the Cards app as a competitor, but does not view it as a substantial threat to Sincerely’s business. In part because, he asserts, gifting is not Apple’s core business focus.

“I don’t think it’s a new direction for Apple,” he says. “I don’t think they’re going to make an amazing gifting experience or offer amazing products like Postagram…I think it’s kind of a checkbox for them.”

Meanwhile, Sincerely will forge ahead with what Brezina believes to be the best mobile gifting experience available. He points to Postagram’s high ratings on the App Store and Android Marketplace, and its appearance in a series of Apple ads as proof. Brezina would not disclose the size of the company’s user base or the number of postcards that have been purchased via Sincerely apps.

“We believe very strongly that a printed photo is this gift that every person on the street would appreciate,” Brezina says.