买一赠一的促销装---怎么说?

“促销装”的说法是：
Promotion package

“买一赠一”的说法是：
Buy one and get one free
（或） two-for-one offers

“买一赠一的促销装”可以这么说：
【Two-For-One Promotion Package】

One plus one promotion set

你可以直接这样说

buy one get one free