I think it was Clarry who posted only yesterday that he'd been speaking with a Paypal rep and he was told that a photograph of the addressed large letter with the stamp cancelled by the PO would be acceptable as proof of postage for defending an INR claim.

If this is true, it means having the postage stamp cancelled by the PO then taking a photograph of it before it is dropped in the bag for subsequent delivery to the distribution centre. I wouldn't take this as gospel though, and would suggest you ring Paypal and ask them yourself.