What is a permit or an indicia
A postal permit allows companies to send mail without putting actual postage on the letter. Instead, they print an indicia. The indicia is what appears in place of the stamp that says something like "Postage Paid - First-class Mail".
You can probably find many examples of this by looking through your own mail. Some have real stamps on them, some have a postage meter amount and date and many more probably have the indicia.
Below is generally what an indicia looks like: