Maybe it's like a regular deal or maybe not, but I will just post it anyways, since I always see this just at 50% discount on Steam, which is meh, here it has 75% off.

BioShock Infinite Season Pass $5

And the base game is 75% too, for base game is is just a regular offer, but is not regular offer for the Season Pass.
The base game is on sale too if it's worth $7.5 to you
on GMG too here you can use your credit

Can I activate it or use it on PC because it says Mac?
YES, also it doesn't matter if this store sells MAC games, the key you receive is a Steam key and can be activated in no matter what OS you have, and you can play the game on Windows or Mac, because game is Steamplay.