That quote is one of the more interesting ones that is credited to Audie. It gives one a lot to think about. Since he was occasionally shown either drinking or smoking in his films and TV appearances, did he mean that at least he didn't get paid for making commercials promoting the habits or did the phrase "100% prostituting myself" refer to how he may have felt about earning a living that started with his fame following the public revelation of his military career. We know that thought bothered him because he referred to the relationship between the two careers often, but how much did it bother him? We will probably never know. His fans, like all of us, are so happy he turned his military fame into a career in Hollywood, but this quote really makes one think.
I believe the quote's from a longer interview where he first talks about the decline in the quality of the projects offered to him and the size of the potential paychecks involved, and compares himself, with some dry humor, to an "over-the-hill prostitute." So he's saying that he draws the line at scandalizing his younger fans with liquor/smoking endorsements. (I believe the Quiet American, which flopped, was the only role that required him to smoke on screen, and most of his characters who are willing to drink are portrayed as flawed or misguided people-Jesse James in Kansas Raiders, Cash Zachary in The Unforgiven. Cases where the film is pretty clearly saying "Kids, don't grow up to be like this guy.")
He seems to have been extremely wary about merchandising deals-the only print ads I've seen him do involved menswear of some kind, or guns, (in other words, things that he would probably have owned and used personally) and one "hope chest" ad involving him and Wanda when they were still dating. He did do some promotional work later in his life (see the recent Isaacson & Slockbower thread) but it appears to have been mostly what we would call "business to business" or maybe "business to investor" rather than aimed at the general public.
He was profoundly ambivalent about the movie business. It's safe to say that he enjoyed the money and some of the perks, and found his share of friends in the film industry, but he did not like the movers and shakers, and he generally wasn't impressed with the finished product. He said once in the last years of his life that the reason he tried to stay in the film business (with A Time For Dying) was that he was too old a dog to learn new tricks.