The makers of several iOS applications have received cease and desist letters over the use of in-app purchasing, from parties who claim the feature infringes on an existing patent.

Computer LogicX, which makes Mix and Mash and Mix and Mash LITE, as well as developer James Thomson, who makes PCalc and DragThing, have received letters threatening legal action if the feature is not removed.

Apple's icon for in-app purchasing.

In Computer LogicX's case, the U.S. patent is No.7,222,078, which Macrumors discovered is owned by holding firm Lodsys. That patent, titled "methods and systems for gathering information from units of a commodity across a network," was acquired by the firm in 2004, and deals with the set up and completion of a transaction.

A snippet from the patent attempts to sum up what the invention is about:

Rob Gloess of Computer LogicX weighed in on the matter in an e-mail to Macrumors, saying the patent holder was taking aim specifically at an upgrade mechanism that involves in-app purchasing. Users hit a button in a free, "lite" version of a Computer LogicX app to download and pay for the premium version.

"Our app, Mix and Mash, has the common model of a limited, free, lite version and a full version that contains all the features," Gloess told the site. "We were told that the button that users click on to upgrade the app, or rather link to the full version on the app store, was in breach of U.S. patent No. 7222078. We couldn't believe it, the upgrade button!!"

Lodsys did not immediately respond to a request for comment as to whether it is cautioning other developers on the use of the technology. And it's not clear why, exactly, the firm chose to single out Computer LogicX (and perhaps Thomson). Thomson hasn't officially named the party who sent him the cease and desist letter, but he hinted it might be Lodsys. The developer says he's waiting to hear back from Apple on the matter before making the information public. Apple did not immediately respond to a request for comment on the situation.

Apple introduced in-app purchasing in early 2009, and later that year enabled the feature to work inside of free applications. The feature was designed to add more flexibility to app upgrades, as well as simplify the process of publishing two versions: a free and a paid, into a single free version that could be easily upgraded with additional functionality.

Developers can design the button or the user interface to enable the upgrade in any manner they see fit, though a pop-up notification that describes what you're purchasing, and an account-confirmation mechanism that allows you to actually make the purchase with your iTunes account, are a part of iOS itself, making it unclear why these infringement claims are not being aimed at Apple, which makes the OS and maintains the payment system.

This is not the first patent-infringement pressure Lodsys has applied. Earlier this year, the firm took aim at a number of tech companies, including Hewlett-Packard, Lenovo, Lexmark International, Novell, and Canon U.S.A., over a patent pertaining to printer software.