I am going to replace some of the structural supports with flimsy material to teach the customer a lesson.
That seems unsafe. Are you sure you really want to cause such damage?
I've been doing this at home.
Intentionally sabotaging a building by using weak building materials is very wrong since it can lead to injuries and even the death of others. You should try to complete the job as best as you can and within specifications to prevent any mishaps from occurring in the future. If there is an issue with the customer, you should work through the legal system and not through trying to get revenge by sabotaging the project.
Don't worry, I will be sneaky about it.
I am worried. People could get hurt if you use shoddy material in a construction project. Like lots of people, it is not worth it, to get back at someone.