As long as they give me receipt, with their company name on it, should be no issue as far as warranty.
The guy who put my last roof on got tired of dealing with paying suppliers, having customers checks bounce and being on the hook for materials and having to provide lien releases* so he would get the material price from the supplier, but the financial transaction for the shingles was directly with the supplier at time of delivery. Then I paid the roofer for his labor separately . At least here, most roofers limit their warranty on labor to 5 years. Besides, if the shingles fail, their warranty is from the manufacturer, not the installer. They may have their own roofers to honor warranty claims, and given that most shingles have a 25 to 50 year warranty, there is a pretty good chance the installation company will have closed, and the owner retired decades later when you have a claim.
The contractor who replaced my driveway had a credit, check and cash price, each a bit lower than the first. But he gave a receipt.
* You should get a lien release in every instance where you have a contractor buy material for your project.