For no reason he can figure, adman Josh Redmont has been receiving checks for $1,000 in the mail, one per month from ''United States Agent,'' for seven years. One day, a stranger walks up to him and says, ''You are now active.'' Guileless, almost-innocent Josh (he has kept the dough, after all) is abruptly plunged into a plot to assassinate a visiting foreign dignitary. Money for Nothing shows the versatile thriller master Westlake in medium-farce mode: perfect-pitch dialogue rendering a silly plot credible, plus a climax featuring a tall, naked woman escaping on a golf cart. Someone comments that she won't get far. His colleague murmurs: ''You just mean on a golf cart. Naked, she actually might get pretty far.''