The verb "send" has two objects: the thing that is sent, and the person who receives it. (Also called "the direct object" and "the indirect object".) "They sent the wrong model to us" has two possible passive sentences: "The wrong model was sent to us." and "We were sent the wrong model."

Here is a source for Peachey's excellent explanation. The verb "send" takes two objects and either can be the subject of a passive voice sentence. https://learnenglish.britishcouncil.org/english-grammar-reference/active-and-passive-voice [excerpt] We can use the indirect object as the subject of a passive verb: [active voice] Someone sent her a cheque for a thousand euros. [passive voice] She was sent a cheque for a thousand euros.

In English, always capitalize the first word in every sentence. In your example, "we" is the receiver of the action not the actor. Therefore, this sentence is stated in the passive voice. You're right, "were sent" is used when something is sent something by someone else. "We" discovered the error but "we" didn't send the package; someone else sent it. .