Turns out to be not a scam. Well, not aimed at me, anyway.
Some time ago I ordered an Ebay item which was pretty flat. It never arrived and the vendor said they'd shipped it first class post as a large letter. I eventually got a refund and, surprise, this is what the item requiring a fee to be paid was.
I asked what the problem was with it and the chap said it was too big. It's one of those grey plastic post bags and on returning home I measured it and found it to be exactly the right size for large letter. Not a surprise, really, since it's designed for Royal Mail's shipping sizes. Anyway, the postage was paid with a first class stamp, which I figure to be worth 68p instead of the £1.05 a large letter requires.
I've now offered to ship it back to the vendor at their cost. (I had to replace it, of course, and found a different source at a fifth of the price, so it's no use to me now anyway.)