parcel; postal package
Refers to a package sent through the postal service. The term is straightforward and commonly understood.
A parcel arrived.
I'd like to send this parcel to the United States.
小包 is a general term for a package or parcel, while 郵便小包 specifically refers to one sent by mail.
宅配便 refers to courier or home delivery services (e.g., Yamato, Sagawa), whereas 郵便小包 is specifically a postal parcel.
Compound of 郵便 (postal service) and 小包 (small package).