Punctuation: comma after introductory element

Sometimes we assume that the comma after an introductory phrase or clause is optional--no big deal to leave it out. But this sentence proves that sometimes leaving it out creates real problems:

If a store patron is attempting to steal the shopkeeper should be able to determine if a theft was committed by asking the suspect questions.

I like to use a comma after my introductory elements. I know this is "heavy" punctuation and that college writing teachers often don't require the introductory comma. But college teachers are usually teaching students to write fiction or essays. I'm teaching a form of technical writing.

I want the introductory comma.