Okay, there are worse things in the world to be concerned about, but this nags at my conscience: when you receive a letter in the mail, and the stamp hasn’t been cancelled, is it permissible to cut it off the envelope and reuse it?
Someone who reuses an uncancelled stamp is availing himself of services for which he has not paid. It’s a form of petty theft. So yes, it’s unethical. It is also illegal.