James Thomson, the owner of TLA Systems, makers of DragThing and PCalc for Mac, and PCalc for iOS, was served with a letter this morning threatening to sue for his use of Apple’s in-app purchasing system.

[ad#Google Adsense 300×250 in story]Thomson didn’t say who the company was that sent the letter, but on Twitter this morning he said, “They seem to be effectively claiming the rights to in-app purchase, but going after me, not Apple.”

Thomson uses in-app purchasing in the free PCalc Lite, allowing customers to upgrade to the full version within the app. His use is no different than what many other developers are using on the App Store.

Thomson was told in the letter that he had 21 days to license the technology from the unnamed company or face a patent infringement lawsuit.

All of the information was sent to Apple, according to Thomson. He has decided not to speak too much about it until he hears back from Apple.