In the US, businesses can pay a fee for a permit that in turn allows them to use precancels at reduced rate.
I have a personal permit which allows me to use pre-cancelled stamps, but I get no discount from the standard rates (i.e., if I am mailing a 1 oz letter with precancels, I have to use $0.49 in postage. My permit also allows me to cancel/postmark my own mail.
In neither case (business nor personal) can you simply "...drop it in the mail". Rather, you have to take your mail down to a designated post office that processes pre-cancelled mail. In my case that is the Albuquerque Business Mail Entry Unit (which is adjacent to the Main Post Office in ABQ). This is how the PO guards against reuse. If you post a letter with precancelled stamps in the regular mail stream, it will/should be rejected for not having proper postage.
Here is a copy of the letter that I got from the PO after completing the process.
