Toni Braxton -- Sued Over $57,000 TV Poster SNAFU

Toni Braxton is responsible for over $57,000 in STOLEN ADVERTISING for her TV show "Braxton Family Values" -- so says a new lawsuit ... and now, the singer's being sued BIG TIME.

A company called National Promotions & Advertising filed the lawsuit in L.A. County Superior Court, claiming it entered into an agreement last year with the producers of Braxton's WEtv reality show -- including Braxton herself -- to promote the show's 2nd Season.

According to the lawsuit, NPA agreed to post 4,300 "BFV" posters around the country in exchange for a fat paycheck -- $57,322.29.

NPA claims it completed its end of the bargain in November -- days before the premiere of the show's second season -- but Braxton and her producing partners didn't pony up.

NPA says it's been trying to get paid ever since ... in vain ... and now, it's suing for its money ... plus damages.