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1·As the package of sample is overweight, airmailing would be too expensive.

由于样品包装过重，空邮费用太高。

2·In order to promote business between us, we are airmailing you samples, under separate cover, for your inspection.

为促进双方业务往来，另封航邮寄上样品，供你参考。

(1)airmail letter航空信

(1)registered airmail航空挂号信；挂号空邮