Unopened invitation to the Apollo XI launch.

My great uncle use to work at Cape Kennedy. He received this invitation to attend the launch for Apollo XI. He sent the unopened invite to my mother ( his niece) and told her not to open it, that it would be worth something someday. Up until a few years ago, we (children) didn't even know what he sent, only that he had sent something in an envelope. My mother has since passed on and we still have it, unopened. It appears to be a printed postcard. We have only tried to view it using a bright light.