Buy one get one free:買一贈一;買一送一。
This shirt is on sales promotion:
buy one and get one free.
這件襯衣正在促銷,買一送一。
Buy one and get one free;
available upon purchase.
買一送一,即買即送。
Those goods are "buy one get one free".
這些商品都是「買一送一」。
I've a coupon for buy one
water melon get one free.
我有一張購物券是「買一個西瓜另送一個西瓜」。