Buying and replacing the special mugs, which can reheat liquids aboard air refueling tankers in flight, has cost the Air Force $326,785 since 2016, Air Force Secretary Heather Wilson said in a letter.

The letter, dated last Wednesday to Sen. Chuck Grassley, R-Iowa, came after Grassley questioned “yet another report of wasteful spending in the Department of Defense” in an earlier letter.

In an Oct. 2 correspondence, Grassley asked Wilson about a Fox News report that found a squadron at California’s Travis Air Force base had spent $56,000 on the metal mugs in the past three years alone, which service members kept dropping and shattering.