I'm going to leave a letter along with a peanut butter bar pastry asking her to keep up with her dogs poop in the backyard
Why do you want to leave a letter like that?
I just want her to pick up her dog's poop.
It sounds nice to leave her a treat along with the letter. It's a good idea for you to let her know about the poop issues caused by her dog.
I tried just talking to her already, but she didn't pick up the poop at all.
Well I hope it works and she starts doing a better job picking up after her pet. It is good that you are communicating with her about the problems instead of trying to get revenge.