An interesting question. A bag usually holds a lot of separate items loosely packed- a woman's hand bag, a bag of donuts etc. A package is usually something solid and tightly wrapped, or boxed.- frozen peas are usually a solid lump so perhaps that's why they are in a package. A pack can be something carried on you like a backpack filled with things you need. or it can be a tight group of the same things - like a pack of dogs or wolves. A pack of cigarettes fits both meanings.