If your pockets aren’t deep enough to fight a corporate giant, then sue the little guys for milk money.

That’s the idea behind a patent company’s legal threats against several independent iPhone app programmers rather than Apple.

Several iOS programmers on Friday morning said they received a legal complaint from Lodsys, a patent-holding firm.

Lodsys is accusing the developers of infringing a patent related to the usage of an “upgrade” button that customers can use to upgrade from a free version of an app to a paid version, or to make purchases from within an app.

Apple provides the payment technology that programmers embed inside their apps, but the Lodsys complaint is instead aimed squarely at the programmers using Apple’s in-app purchasing system.

Programmers who say they have received the complaint include James Thomson, creator of the scientific calculator app PCalc; Dave Castelnuovo, creator of the best-selling game Pocket God; and Matt Braun, developer of the popular iPhone kids game MASH.

Many apps use Apple’s in-app payment system, so the number of companies to receive the legal threat could soon grow much larger.

“Just got hit by very worrying threat of patent infringement lawsuit for using in-app purchase in PCalc Lite. Legal docs arrived via fedex,” Thomson said on his Twitter account Friday morning.

Programmers who have received the complaint say that Lodsys is demanding that they negotiate for a license to use the “upgrade” technology within 21 days, or a lawsuit will be filed.

This incident is an example of a practice that many in the industry would call “patent trolling,” which means using patents for little purpose other than to sue other companies until they cough up damages or licensing fees.

Lodsys did not respond to a request for comment.

On its website, Lodsys claims ownership of patents related to technologies that “provide for online purchasing of consumable supplies,” and “sell upgrades or complimentary products,” among others.

Lodsys is based in East Texas, which is home to a federal court that often sides with patent holders. Patent lawyers around the world know that the easiest and quickest way to win a patent-related dispute is to file the complaint in Marshall, Texas. As a result, many defendants choose to settle instead of fight.

“One concern is that if we are the lightning rod, it could end up being pretty expensive for us because they would choose to sue us no matter what our argument is,” Pocket God creator Castelnuovo told Wired.com.

Apple did not respond to a request for comment. It’s unclear whether the company will get involved in the patent dispute.

However, it seems likely that Apple will intervene. If Lodsys sues or imposes licensing fees on iOS programmers, it would deter developers from building apps for the iPhone, iPad and iPod Touch, and would hurt the ecosystem as a whole. Also, Apple takes 30 percent of each in-app sale, so it would lose money if Lodsys were to succeed.