UK: Do you have to post as “Signed For”? I have sent 10 orders in the past week which have been sent standard Large Letter, all of them are tracked as delivered but Amazon is not accepting them as valid tracking.

My valid tracking rate is 0% out of 10 orders. Every single one is marked as delivered on Royal Mail website and the tracking has been uploaded to each order. Do I have to send everything as Signed For? Even though they are being scanned as delivered?