Just picked up an item from the Royal Mail delivery office, paying £2.50 for the privilege because the sender used a counterfeit postage stamp.
The sender is an older relative, with some vulnerabilities and would be devastated that her kind gesture caused this inconvenience. She is definitely not a person who would deliberately buy fake stamps, or buy something from a recognisably dodgy source.
Attached a picture to show how good the fake is. The guy at the delivery office said that one way to tell is that the barcode part will feel rough when you run your fingers over it.