USPS's First Thermochromic Ink stamp was released on June 20th, 2017 to
commemorate the August 21st Total Eclipse of the Sun. The stamp has a black circle at the middle, when it is exposed to light/heat the image of the moon will appear. Thermochromic inks are vulnerable to UV light and should be kept out of direct sunlight as much as possible to preserve this special effect. Since the stamps on cover are travelling long way, they are exposed to sunlight and obviously they lose its special feature. In order to preserve the Mint stamps USPS is providing some special envelopes at an extra cost.
Thanks a lot Arakan, USA for sending these stamps on cover.