荷物 and 小包: baggage and wrapped parcels
Compare the broad category of goods to carry with a parcel sent as a package.
Goods to transport
荷物 (にもつ) means baggage or goods to carry or store. 「荷物を車に積みました」 means loading baggage into a car.
「荷物が三つあります」 counts three pieces of baggage. It does not count the individual objects inside each piece.
A parcel to send
小包 (こづつみ) means a wrapped parcel. 「小包を受け取りました」 means receiving a parcel.
「小包を開けると、本が入っていました」 says a book was found inside when the parcel was opened. Separate the package from its contents.
Different names for the same item
A delivered parcel can also be broadly called 荷物. The two words do not always identify separate objects.
「小包は届きましたが、まだ開けていません」 says the parcel arrived but has not been opened. Arrival and inspection are distinct.
Does 「小包は届きました」 establish that the contents were checked?
Show the answer and why
No. It only reports arrival; opening and checking are separate actions.