you gave information that meant it had something of value that they're allowed to accept only by SD?
Is there anything that can only be accepted by SD? A £70 item can be sent by signed for, but you only get £50 if they lose it. If you send cash by signed for, you get no compensation for loss but you do get 8 stamps for the failure to obtain delivery confirmation.
If the old stamps were genuinely invalid, then things posted with them on would be treated as completely unpaid; however, from what I'm told, they just try to add a flat rate surcharge, irrespective of the actual postage concerned, to be paid by the recipient when delivering the item.
https://www.royalmail.com/receiving-mail/pay-a-fee says that for letters and large letters, you will be surcharged £1.10 for using a non-barcoded stamp**, but £1.50 for insufficient postage and £2.50 for no postage.
The fee for small parcels is the same regardless of what stamps, if any, are used, while for medium parcels you have to pay £1.50 plus the unpaid amount.
So if the recipient is happy to pay £1.50, then sticking a barcoded 1p stamp on a 251g-750g large letter appears to be cheaper than paying £1.85 to £3.30 at a post office/online.
It's not clear what happens if you stick on a non-barcoded 1p - will it be regarded as no postage or insufficient. It seems illogical to only charge £1.10, when using a valid 1p stamp would incur a £1.50 charge.
**Let's ignore that plenty of non-barcoded stamps are still valid
No, but I assume it's a unique identifier for that stamp. So as with barcoded train tickets, when scanned the fact that the stamp has been used can be logged, and any repeat use detected. And as with barcoded train tickets there's no need to physically clip (train tickets)/postmark (Royal Mail) to show that it's been used. So no more need to have physical postmarking machines - which in turn means less to go wrong, and less chance of damaging fragile post.
It also means an end to letters headed 'date as postmark' in that there won't be a postmark any more, although I suppose there may still be franked mail.
That is the theory, but my stamp dealer is selling unfranked barcoded stamps for amounts that make it worth the time to sort them. People also buy franked barcoded stamps, and they are still buying franked invalid stamps in bulk too.
As an experiment, I received a large letter with an unfranked barcoded 1st class stamp. I took out the item and resealed the envelope. I have then brought the envelope to several places in Scotland and northern England and posted it back to myself, and it has come back to me 5 times. On the 5th delivery the postman drew a line through the stamp. (Note that the postman would not have recognised that particular envelope because I buy from the same seller frequently and the packages are always in the same style.)
