I would strongly warn you against this, in this day & age.

Back when we made margin on products, everyone could do this. Plus we used brands that were reliable to begin with. If something went bad, it was covered. There was enough margin in the product, that no one was sweating it.

Now a days = Everything under the sun seems to have problems now & then. No matter what brand it is. And because of manufactures and the internet, we make very little margin on anything.
Are the manufacture paying you to warranty their crap? Hell no! And they have taken away margin on us to do so, even if we wanted.

Policy has been for awhile now - Time & Materials. Even if we supply it. If they want to buy it themselves, fine. They are responsible for making sure a working product is there when we come to install. If something's missing or screwed up, they are still paying for our time. If we provide it, we at least take care of that part (that is what the "small" markup is for). But even 20 days after we put it in, if it fails (even if we provided it), they still are paying a service fee.

To do work for free for greedy manufactures, is a quick way to put yourself out of business these days.