because I am a private seller, I no longer have the receipt from Post office
What does that have to do with being a private seller? The rules are the same for all sellers.
PayPal gives buyer up to 180 days - that's six months! - to claim that an item didn't arrive. Or a credit chargeback can be issued even weeks or months after the sale.
It's important for all sellers to keep their postal proofs, to qualify for eBay or PayPal seller protection. It isn't difficult, just keep them in a file or an envelope for at least six months.
