From the footnote on p. 79 of the 1985 paperback edition of Carl Sagan’s book Cosmos:
There are many unexpected developments in mustering spacecraft to explore the planets. This is one of them: Among the instruments aboard one of the Pioneer Venus entry probes was a net flux radiometer, designed to measure simultaneously the amount of infrared energy flowing upwards and downwards at each position in the Venus atmosphere. The instrument required a sturdy window that was also transparent to infrared radiation. A 13.5-karat diamond was imported and milled into the desired window. However, the contractor was required to pay a $12,000 import duty. Eventually, the U.S. Customs service decided that after the diamond was launched to Venus it was unavailable for trade on Earth and refunded the money to the manufacturer.
This reminds me of the customs form the Apollo 11 crew filled out upon their return to Earth.