Lust Was The Payoff For This — Passion Pair
From the back:
PASSION PARADISE…
… that’s Puerto Rico, and the lush surroundings of the plush hotel add to the boredom of the idle rich… of people like Paul and Elaine Combs. It is so easy then, for determined extortion artists to move in and mark them as their next passion victims. The blackmailers are Jim and Lois MacIntyre, smooth and polished in all the tricks of their trade… and their trade is love… until the pictures are all developed. Then the shame and degradation of their victims becomes reflected in a negotiable check… the only true sin money…
In this case Don Elliott is a pseudonym of Robert Silverberg.