This is a pre-paid envelope created for a ham radio DX expedition to Juan de Nova that took place between March 29 to April 11. They used this to mail QSL cards back to those who had contacted the expedition.
I don't know much about HAM radio, but I believe there is a branch of the hobby that involves trying to reach far away remote stations. It's known as DXing. After contact has been made, the HAM operator can request a confirmation card known as a QSL card, acknowledging that contact took place.