I have a UK one on my keyring right now, it's from Barclaycard. It reads "Finder, please deposit in any post box - no stamp needed", and also has an 0800 phone

I have a UK one on my keyring right now, it's from Barclaycard. It reads "Finder, please deposit in any post box - no stamp needed", and also has an 0800 phone number on it. It's never been tested, but I like to think it works!