I don't understand why we can't use the Passé Composé

Question

I don't understand why we can't use the 
Passé Composé
when there is no reference to a duration of time. For example "during that year the letters had been sent"??? This sentence seems like it was a one time action. ???

Jim Jones · Answer

Bonjour Ana K"during that year the letters had been sent"Without reference to a particular kwiziq lesson or story where there may have been some further context, I see this as an activity that used to be the case.That suggests to me a report of a background historic situation only which is why it is in the imperfect.This is why regretfully I am unable to agree with your proposal.Bonne journéeJim

Cécile Janty-Davies · Answer

Hi Ana, 
To add to what Jim has said, without the specific context, it is not possible to give a clear answer.
Could you point us to the specific exercise where you saw that sentence?