Offices need mailing supplies. Is it legal to go to the post office and just take it? We don’t see why not, since you will be paying for the supplies anyway when you use them for mail. By the same reasoning, taking supplies for purposes other then using them for mailing is in most likelihood illegal.
It’s best to not be impolite, and take only the supplies you actually need and leave some for other people. It’s also a good idea to ask a postal employee for permission.