Agreed. I would never pay anyone to do a job that I could carry out on my own for free, and most likely, with more potent results. (I eat a lot of gas station bean burritos.)
I'm wondering, though, is there any way for them to find out? If the poo smell were potent enough, would that be a sufficient reason for the postal workers to open the letter and find the poo dollar? Also, I heard somewhere they sometimes let dogs check the mail for drugs. And whenever I take my dog for a walk I always end up having to forcibly drag him away after he's sniffed a foreign dog turd for about 6 hours, so my guess is that if a dog were to sniff out the mail for drugs, I would be screwed.
But I'm not sure exactly how much trouble I would get into. Knowing how smelly my shit is, I'd probably be taken in for mass murder, and possibly a violation of the Geneva Convention.