Keep the Receipt Where the Thing Will Break
The blender coughs once, smells hot, and stops. Someone remembers that it might still be under warranty. The receipt may be in an inbox, unless it was printed, unless the store account used an old email address, unless the purchase was made by the other person in the house. By the time the search becomes annoying, a replacement blender has begun to look cheap.
Households lose money on products that fail twice: first when the item stops working, and again when the proof of purchase cannot be found. A receipt will not turn every breakdown into a free repair. It does make the next question possible while a return period, store policy, manufacturer warranty, or card benefit may still matter.
You do not need a filing cabinet full of grocery slips. Keep a short record for purchases whose failure would bother the budget: appliances, electronics, tools, furniture, luggage, children's gear, and anything else expensive enough that you would make a claim instead of shrugging.
The useful moment is the day the item enters the house. Photograph the receipt, the model label, and the serial number. Save the order confirmation or invoice as a file. Write the purchase date, seller, price, and the last day of any return period you intend to use.