Chuck Noland: I couldn't even kill myself the way I wanted to. I had power over nothing.
Chuck Noland: Aha. Look what I've created. I have made FIRE.
Chuck Noland: You wouldn't have a match by any chance would you?
Chuck Noland: I should've never gotten on that plane. I should've never gotten out of the car.
Chuck Noland: That's a search area of 500,000 square miles. That's twice the size of Texas. They may never find us.
Kelly Frears: You said you'd be right back.
Chuck Noland: I'm so sorry.
Kelly Frears: Me too.
Chuck Noland: Gotta love crab. In the nick of time too. I couldn't take much more of those coconuts. Coconut milk is a natural laxative. That's something Gilligan never told us.
Answer: No one knows what was in the package. I think Chuck doesn't open it in order to keep his sanity. The package is his only link to his life before being stranded, working for FedEx and making sure packages get delivered. He was determined to make sure it got delivered just as he was determined to survive and get back to civilization. Success in one meant success in the other. While Zemeckis has joked there was a waterproof satellite phone in it, the real answer from the script is, no joke, salsa verde. Its a care package with a note imploring Bettina's (the artist) husband (naked cowboy) to come back and spice up their life like the salsa.