Here are a few questions to keep in mind, since the answer to each can affect the ruling.
Did you sell your chametz with your rabbi?
If the chametz was in your possession from before Pesach and you sold it through your rabbi, then in all likelihood the chametz item was included in the sale as well. Although the chametz was not locked up in the cabinet as it should have been, it was still included in the sale since there is usually a statement in the contract that all chametz is considered sold regardless of its location. Please check with your rabbi. The chametz should be pushed with a broom or a pole into the closet; do not use your hands, since we are concerned that if you pick up the chametz, you might forget and eat it.
Was the chametz found on yom tov or Chol Ha’moed?
Chametz found on Shabbos or yom tov is muktza, and may not be moved. This presents a problem, since you are not permitted to dwell in a room that contains chametz out of concern you might forget and eat it. You must place a bowl or pot over the chametz as a reminder. If the chametz was found on Chol Ha’moed, and you sold your chametz with your rabbi, the chametz may not be directly picked up and placed in the cabinet but should instead be kicked or pushed with a broom until concealed.
Was the item purchased on Pesach?
If the chametz item was purchased on Pesach or even on erev Pesach after the chametz sale, it must be burned at the first opportunity. As explained above, it cannot be moved, and it certainly may not be burned on Shabbos or yom tov. Instead, it should be covered with a utensil until after yom tov, at which point it must be burned. If the purchased item was only kitniyos (does not contain any of the five grains — wheat, barley, spelt, rye or oats), such as a bottle of corn oil, it may be put away until after Pesach. A kashrus expert should be consulted, because some might contain chametz even though it is not obvious.
What do I do when a package that included chametz was delivered on my porch on Pesach?
You may not leave chametz on your property even if you have no intention of acquiring it. If the delivery took place on yom tov, you must cover it until after Pesach. On Chol Ha’moed you must take a broom or a pole and push it off your property. You may not pick it up with your hands. The Mishnah Berurah writes that just picking up chametz on Pesach even without intent to acquire it is forbidden. You are only permitted to pick up chametz with the intent to burn it.