The Kennedy Center is returning mail orders for "A Chorus Line" tickets that were postmarked after noon yesterday, but the show is not sold out, as far as the Center can tell. Box office and Instantcharge sales will open on Aug. 28, according to a Center spokesman, and the interim time is needed in order to process the mail orders already received.

As of noon yesterday, 18,684 amil orders had been received, most of them requesting more than one ticked. An additional 20,000 tickets have been allocated to Theater Guild subscribers, though some of these may be returned to general sales if the Guild membership drive does not reach the 20,000 mark before the end of the month. The current Guild membership figure is around 15,000.

The mailorder deluge is unprecedented at the Center, spolesmen say. The show will open here at a Sept. 14 benefit for the Juvenile Diabetes Foundation and play for eight weeks. Total capacity for the run is approximately 140,000.

Why such hoopla for"A Chorus Line?" Kennedy Center chairman Roger S. Stevens can't figure it out, "I don't understand the draw of that show, frankly," he said. "I'm not one of its great supporters. But that doesn't make any difference."

The show's publicist is full of reasons for its success. "The cast is all young, beautiful people," he said. It's "exquisitely lit, the score is written to fit the book," and the book is "sort of everybody's story," about "every young person who's ambitious to get ahead." He thinks "A Chorus Line" is "the show of the century."