A service contract is rarely one piece of paper. A receipt, a warranty card, an install record, a written estimate, and a signed contract each prove a different thing, and each tends to go missing at a different moment. Set up one folder the day the item arrives and file each paper as it comes, before any question is raised.
The papers to keep
Keep the proof of purchase, which shows what you bought and when. Keep the model and serial number as they are written on each paper, and photograph the rating plate on the item itself so the two can be matched later. Keep the installer's record if someone else installed the item. Keep any warranty card and note whether it must be returned to the maker to activate the maker's own term. Keep the signed contract with every page, not just the last one.
What each paper proves
A warranty card is a form from the maker. It records what the maker says about its own product, and that promise may be separate from the one in a service contract. A written estimate is a quote for work, not a settled promise about what a later claim will pay. A signed contract records what you and the seller agreed. Keep each paper in its own sleeve so you can show exactly which document says what.
Notes on calls and messages
Log each call to the seller or the administrator with the date, the person's name, and any reference number. Email anything that matters and keep the sent copy. Save any text message as a screenshot. This trail costs nothing to build and is often the only record that survives a dispute.
When to pull the folder out
Open the folder before you accept a renewal notice, before you agree to a transfer, and before any repair starts. Compare the model and serial on each paper with the item itself. A mismatch found early is a clerical fix. Found after a repair, it becomes a dispute.