I thought I read somewhere that this is part of Delta's new TTM. Diamonds receive complimentary tray tables at check-in. Platinums, are allowed to select tray tables, if available, 5 days in advance of departure, Golds 3 days, and Silvers day of departure. During check-in you may be offered the opportunity to purchase a tray table for use during your flight for a small, nominal fee, depending on the length of your flight and the in-flight services offered.