One issue has been that of postal covers carried on the mission by the crew. NASA has authorized astronauts to carry certain personal items on manned flights, known as "Astronaut Preference Kits" (APKs). All such items must be listed and approved prior to launch, are intended for private use or as personal gifts after the flight, and may not be employed for commercial purposes or personal gain. Astronaut kits typically have included badges, jewelry, coins, medals, flags, stamps, postal covers, charms, currency, printed materials, pennants, and similar easily packed, lightweight, and non-hazardous mementos. On Apollo 15, there were a total of 641 postal covers carried by the crew, of which 243 were listed and authorized before the flight and 398 were not.