If x copies are sent by mail for the first time, 200-x copies will be sent by mail for the second time. According to the situation that the first mail order was less than 100 and the second mail order was more than 100, the price equation was solved twice.

1.6x？ ( 1+ 10%)+(200-x)？ 1.6? ( 1- 10%)=304,

Solution: x=50,

200-50= 150.

Answer: 50 copies were ordered by mail for the first time and 150 copies by mail for the second time.