I've sold hatching eggs and bought hatching eggs plenty of times. Never, ever, ever guarantee hatch rates. Once the box is open at their house, all further responsibility for the eggs is on them.
Valid complaints look like;
"These eggs were filthy and looked like they got rolled in mud before they shipped", "Half the eggs you shipped me were cracked", "The eggs were packaged poorly so they were rolling around in the box", "The chicks or eggs weren't the variety/breed/color/quality listed" or MAYBE even "All of the air cells were detached". But never, ever, hatch rate.
What if they set the incubator wrong? What if their incubator has an error and runs 2*F too high or something. A difference that small can destroy a batch of eggs and has nothing to do with you. I had a lady who had her cat fall asleep on her warm incubator and the temperature spiked to 104 for half a day and only one hatched. Not your fault. What if their power went out and they sat heatless for 8 hours. Not your fault.
A simple reply looks like;
"I'm sorry, but we only guarantee that the eggs are fertile, clean, fresh and will arrive safely at your door. Due to variables outside our control we cannot offer returns or replacements based on hatch rates.
Here is where our policy on hatching rates is stated on our purchase page.
*insert screen capture here*"
And that should be the end of it and don't think on it again.