Senatus Consultum
You've secured a decree of the senate (senatus consultum) favoring a law lowering grain duties and prioritizing grain shipping.
What's the next move?
Take the law granting special benefits to grain merchants to the tribal assembly
Make a speech from the Rostra explaining to the people the benefits of the law you want to propose.
Nothing; you've done everything you needed to do.
Tell the grain merchants they can start ignoring standard duties at the harbors