It sometimes is difficult to commit to decisions regarding what to give away to charity, because the items usually stay in our homes for a while. So, it is possible to remove the items from the box or bag, and keep it. However, you probably put it there for a reason. To make sure that donations stay donations, it can be as simple as getting a box that closes at the top. Fill it, and then seal the box with packaging tape. Label it as to its contents, and take it in soon.
I like to give to people that are giving the items away, this works best for clothing and bedding, but also toys and children's books--children always need toys and reading material.