Ugh. If I didn't already have 7 guinea pigs and am at my limit I'd go get her. She looks like a decent little piggy from the picture.
Why would anyone put a guinea pig in a MAILBOX? I guess she thought it would be safe from predators there and would be found fairly quickly. But still, it was left with no food or water and there would have been no ventilation. There is a picture in the paper of the piggy sitting in the mailbox, which is one of those used in rural areas, the classic round top that sits on a post and is the shape and size of many of the small animal carriers you find in pet stores.