How to nicely decline those who use Pee Pads?

DO NOT think it shallow to have this requirement! It is your right as a home service provider to only accept dogs that meet your requirements.

I too do not accept dogs who use pee pads. They need to go outside. Where I live, there are many high rises, and owners who have trained their dogs to use pee pads or go out onto areas on their balconies. Therefore, I am very clear with owners. My primary method is through the Meet & Greet, although that can be a waste of everybody's time when an owner finally admits their dog might use them for "emergencies." I do scrutinize any information that they've provided in their dog's profiles too for hints, Lastly I have added this to my own profile, even though there's no guarantee that an owner is going to read it. It states "your dog must be housebroken (i.e., only potties outside, no pee pads)."

A dog who has been trained to use pee pads for any reason is going to use anything it finds. In my experience, I don't believe I've encountered an owner who lied about their dog's training, so I wouldn't assume that will occur. I have had some suspect ones even though their owners were mortified about the dog's behavior. I realize that dogs will have accidents. You can't totally avoid or prevent. You just have to do your best screening potential clients.

Stand firm. Good luck!