There's a form on there (unfilled), but there's a sticker over it.
The sticker says REF: and then a ref number, consisting of three numbers, a dash, then two letters. Example : 123 - AB
Then :
SURNAME : my surname only. No Mr, no first initial, no first name
ADDRESS : my address
POSTCODE : my post code DATE BOOKED : the date
REASON : THE SENDER DID NOT PAY THE FULL POSTAGE
AMOUNT DUE : £2.00
Then some text :
* All post items entering the UK from outside the EU.....etc etc etc
Then :
What to do next? you can pay in a number of ways : [the Y of "you" is just as I've written it. Uncapitalized]
Then some options :
* Online at http://www.royalmail.co.uk/feetopay" onclick="window.open(this.href);return false; [I checked it, it's a real page on the RM website]
* With postage
* In person
Then...
Please note - if you are planning to pay an outstanding fee on collection, credit / debit cards cannot be processed at Delivery Offices.
Then...
Please note, we will keep underpaid items for 18 calendar day...etc etc etc.
Here is an image of a similar one from google :

That image is not quite the same as the one I got. The one I got also does not have the small print at the bottom, talking about RM's trading name and registered number and address. It doesn't have that. Instead it has it along the right edge. It starts with "P4605 July 2013". It's 2019.
On the other side it has the RM logo at top left, 2 "Affix postage here" boxes top right, it has the real address of my nearest delivery office, and it has the opening times, which I think are correct. It also has a phone number, which, from what I've checked (here : https://www.whatdotheyknow.com/request/ ... r.pdf.html" onclick="window.open(this.href);return false; ) is for that delivery office. But when I googled the delivery office, another number came up, although I think it's the main RM number.
I haven't ordered anything from a company or anyone on ebay nor an amazon seller.
I can't think of any way it could be from the council or gov. Surely they would pay the postage, and I've never heard of them sending anyone something so heavy or large as to require £2 postage. Usually whatever they have to say can be said on 1 or 2 pages.
There's only 1 person who knows my name and address and who might ever want to send me anything in the post. But I think they're smart enough not to make such an error.
Is this a scam?