This is the 9210th most frequent Russian word.
службой
The Russian word "службой" is the instrumental case of "служба," meaning "service" (e.g., work, duty, or service role).
In this sentence, 'службой' refers to a service provision (the postal service) in the instrumental case, indicating the means or method used for sending the package.
Он воспользовался почтовой службой для отправки посылки.
He used the postal service to send a package.
Here, 'службой' is used to denote a specific organizational unit (technical support service), also in the instrumental case, elaborating on the function or role of the department.
Службой технической поддержки занимается отдельный отдел.
The technical support service is handled by a separate department.
In this context, 'службой' denotes employment or duty (military service), again in the instrumental case, as it's part of the phrase indicating pride in one's role or involvement.