I'm thinking of keeping 10% of the discount
What is your reason for wanting to keep 10% of the discount?
I don't think the service was worth the difference.
It is wrong to steal something because you are disappointed. It is to be expected you honor a deal when it is made and if you do not like it do not do business with them again.
I want to make sure my seller knows that I was not happy with their service.
Stealing from someone does not make up for whatever problem or issue you may feel you have. Stealing is wrong no matter what the situation.