Buy Shipping - Scan at the Post office for non tracked large letters?

Hi,
I used Buy Shipping to purchase Royal Mail 1st Class Large Letter on Amazon. There is no tracking just a QR code on the label. It doesn’t say that I have to go to a Post office to scan like it does for Signed For.
Can i simply put the envelopes in the Royal Mail box outside the post office or do I need to get each letter scanned at the cashier?
Thank you