Joan Rivers -- Marketing Genius

Joan Rivers is the MASTER of the shameless publicity stunt ... 'cause her Costco shopping-cart protest didn't just get her all over TV ... it also provided an INSTANT boost to her book sales.

We broke the story ... cops responded to the Costco in Burbank yesterday -- where Joan had handcuffed herself to a shopping cart to protest the store for not carrying her book.

During the protest, sources tell us Joan and her team managed to sell 150 copies to various customers. Since Costco refused to carry the book, Joan was forced to sell them out of a box she kept in the Costco parking lot. Cash only.

And Joan got a big, fat bonus -- multiple national media outlets ... including "Today" and "Fox News" ... LOVED the story and have already aired the TMZ video of Joan's hilarious encounter with police.

Bottom line ... Joan's stupid little stunt resulted in MAJOR exposure to millions of people ... and it didn't cost her a dime.

Not a bad day's work.