What are the guidelines/ regulations per USPS when printing a design on an envelope?

I'm designing stationary for a client, and he wants a border on the bottom of the envelope, however, I think there are some rules when having graphics on the envelope, and I can't seem to find the answers online. I know that the post office prints tracking info or whatever on the left hand bottom corner on the front of the envelope, so I need to know if a border on the bottom is allowed or not. And if it is, under what guidelines do I need to follow in order for it to be substantial for mailing.