Royal Mail have sent me a card through informing me that they can't deliever my item because sender didn't...?

pay full postage. On the card there is a section for them to tell me how much payment is due to enable me to pay online but they have left this section blank. Why have they done this?

Do the words "royal pain" come to mind here, or perhaps just careless?

Because the postman was lazy.

I got one of those as well,Recycle bin is where the card is now.

I got the same. Just go to your local collections office. There's a package/letter waiting for you

That's a scam. Don't pay it!