Leather stretch mark You can warranty you work but that’s it and that has its limitations. You have no idea if this car is go to sit in the Florida sun for six months Or it’s going to be in a place that has 100% humidity you have no idea if he’s putting a leather conditioner or protector on it ...to many unknowns. Leather in sun gets baked. Leather is a totally different animal (pun intended) and don’t worry about those slight stretch marks . If you have done an amazing job and you confident in your work and everything you did was professional in every way then you should not even have the word warranty in your mind.