From torah.org:
QUESTION: What should be done if a package containing chametz arrives at one's home or business during Pesach?
DISCUSSION: One who knows - or even suspects - that the package may contain actual chametz, may not assume ownership of the package. If he can refuse to accept the package, he should do so. If he cannot, he should not bring it into his house or yard and should have specific halachic intent not to "acquire" the chametz. The package is considered "ownerless" - anyone who wants it is free to take it.
If the package was mistakenly brought into the home or business, one must have specific intent not to "acquire" it. One may not touch the actual chametz. O.C. 446:10.
If the package comes on Chol ha-Moed, the chametz should be immediately discarded, either by burning it or by flushing it down the toilet. If it comes on Shabbos or Yom Tov, it should be left where it is [The chametz is severe muktzeh and may not be moved for any reason; O.C. 446:1. Some poskim add that it may not even be moved with one's body or foot, even though other types of severe muktzeh may be; L'ehoros Nossan 5:30] and covered with a vessel until it can be discarded.
While most people do not expect to receive packages containing chametz during Pesach, one should be aware of a recent problem that applies to almost everybody. Many packaged items are insulated by packing pellets that protect the contents during transport. In the past, this cushioning was made from polystyrene, but recently, some companies have begun using biodegradable "peanuts" which are made from edible corn starch or wheat starch. Those that are made from wheat starch may be halachically considered "actual chametz" since they are fit for human consumption. If a package insulated with these "peanuts" arrives on Pesach, the halachos stated above may apply. A rav should be consulted.
A more lenient ruling might be based on the arguement that these pellets have been designated as packing material - not food. They have been processed to remove their nutrients and thus lost their "chametz form" and may be stored on Pesach; refer to O.C. 442:3 and 9, Mishnah Berurah 15, 41 and 42 and Chazon Ish O.C. 116:8. This is a questionable argument and a rav must be consulted.
While it may not always be easy to differentiate between the different types of packing pellets, there is a definite difference in appearance between the polystyrene and the starch ones. The polystyrene ones come in random shapes while the starch ones look as though they have been extruded through the holes of a machine. Each piece is perfectly cylindrical and is water soluble.
