Continuity mistake: When Tom Hanks cuts his hand and loses his temper, he picks up the volleyball with a bloody hand and throws it. When he picks it up, you can see that his fingers are spread. When you see the handprint that he makes "Wilson" with, you can see that the fingerprints are together and parallel.
Continuity mistake: After becoming stranded, Chuck discovers the first package to wash ashore. While he is walking towards it, you can clearly see some of the stretch of beach behind him (as well as some in front of him) This shot is somewhat of a tight one. When the camera "seamlessly" cuts back to a shot of him bending over to pick up the package, you can clearly see that the beach is now covered with dozens of jagged rocks. The rocks would have been seen in the shot before this. But in this case, the shoreline was cleared of them in the first shot. (The beach is not cleared of the rocks; they are underwater. Still a good continuity error submission, but caused by filming the shot at high tide and low tide - it is the same section of beach) (00:32:35)
Continuity mistake: When Tom Hanks opens the FedEx packages he finds the ice skates. When he decides to cut a bandage for his cut leg he uses the skates to cut the material to wrap his leg. If you look at the skate while he cuts the material there is no shoe tie on it but when he drops the skate to get the shoe tie it is back on it. (01:01:35)
Continuity mistake: When Chuck is looking through the wallet of the dead pilot, you can see the corner of a dollar bill, then the shot breaks away, when he takes the photo out, the dollar's disappeared. (00:46:30)
Continuity mistake: When Tom Hanks first drifts onto the island and is asleep his head is facing out to sea. In the next scene when he gets out of the raft his head is in line with the rest of the beach. (00:30:15)
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.