This Purchase order shall be confirmed by Seller's signing on the returning copy of this Purchase Order and mailing the copy to Buyer within three days after Seller's receipt of this Purchase Order.

在售货方开出订购单收据之后， 售货方必须在三天之内，在返回的订购单的副本签字，并把这个副本邮寄给购买方，这样做才能确认这个订购单。

youdao

People opt to send forms and letters via email rather than printing and mailing a hard copy.

FORBES: Tablets Eat Into Lexmark Printer Biz But Not That Badly

Keep a copy of your protest and proof of timely mailing.

FORBES: Everything You Ever Wanted To Know About Fighting The IRS But Were Afraid To Ask

This means a copy of the virus is quickly dispatched to listservs, client lists and internal mailing lists, enabling it to leap city, country and continent borders at incredible speed.

CNN: 'Melissa' May Have Built a False Sense of Security