These days, most “durable” purchases come with a user manual (often written in rather confusing but sometimes amusing Chinglish!) and some form of warranty that requires the buyer to present proof of purchase by way of a till slip or similar. Hint: staple the till slip/proof of purchase inside the back cover of the user guide and file it somewhere safe. This way, you’ll always have the two most important pieces of paper in one place, and retrievable should you need either of them. As a corollary, whenever I buy a warrantied item as a present, I keep the proof of purchase so the recipient can, if need be, put in a warranty claim.