Was Sent or Have Been Sent?

The letter has been sent to our office already. The letter was sent to our office yesterday. Note that "has been" is generally used for a past event with no specific timeframe, whereas if the timing is specified (i.e. yesterday) it would be better to use "was".

Was sent or had been sent?

In English , passive past perfect ( had been sent ) tense is usually use when there is another time frame in the same sentence. Eg. The letter had been sent to you twice before your wife called me. In this example, there were two time frames ( or activities ).

Have been sent or have sent?

Where there's a specific time mentioned then it's generally a simple past: "The goods were sent yesterday/last week/a fortnight ago..." But you can say: "The goods have already been sent", or "The goods have since been sent/now been sent".