There is a slight difference. "You should be receiving your package BY next week" means no later than next week, but possibly this week. "You should be receiving your package next week" means you will receive it next week, but not any earlier than next week. "By" in this context means no later than, but possibly earlier than. Example: I expect to get my Ph.D. by 2020 means I expect to get it no later than 2020, but possibly earlier such as 2019.