The way of saying this in Chinese is different to the English. Translating literally they say "Buy 1, send 1" so I guess you could say they're saying "If you buy 1, we'll send you home with another 1 too".
The Chinese phrase is “买一个送一个”。 [ mǎi yī gè, sòng yī gè ].
You can see at the bottom of the poster it says 买2个，送一个 so that's saying if you buy 2, then they'll give you 1.
*个 can be replaced by other measure words; for example 买一件送一件 when buying clothing.