I had sent you a letter vs I have sent you a letter

Some examples from the web:

• Peter, I sent you a letter.
• Seo In Chan's girlfriend, Han Se Gyeong, sent you a letter.
• I sent you a letter from Kabul... but the dukhs downed our aircraft.
• I sent you a letter before they knew what was wrong.
• Some lawyer sent you a letter.
• King Francis has sent you a letter.
• He mentioned that both the Slovakian and Polish flags were flying at half-mast and that he had sent a letter of condolences to both Heads of State.
• He mentioned that both the Slovakian and Polish flags were flying at half-mast and that he had sent a letter of condolences to both Heads of State.
• The chairman of the JURI Committee had sent a letter setting out the reasons for this decision to the President of Parliament on 16.01.2008.
• A week earlier, he had sent a letter stating:
• As the Honourable Member points out, the Commission had sent a letter to AGCOM on 24 May 2006 following AGCOM's notification of the fixed termination markets.
• The chairman of the JURI Committee had sent a letter setting out the reasons for this decision to the President of Parliament on 16.01.2008.
• Peter, I sent you a letter.
• I sent you a letter, and you never called, not once.
• We sent you a letter asking you to be a character witness for Joe Miller.
• After Warsaw was taken I had an address for her there, then I sent letters to Paris.
• No one was aware that Malcolm had sent those letters.
• His parents sent us a letter.
• Count Dracula he sent me a letter from Transylvania.
• She had sent a letter to Parliament proposing a change to the agenda but Parliament did not find this possible so I will transmit the message from her.

Some examples from the web:

• Mr President, I have already sent you a letter - to which I have not received an answer - and I would like to ask you under which rule the microphone was switched off and why it was relevant to apply this rule.
• I have sent a letter to the Bishop.
• I have sent him a letter, asking for his cooperation.
• So I have sent a letter to the Maltese authorities and the Foreign Minister will visit us tomorrow.
• I have sent a letter to Hitler informing him, as of June 10th Italy will enter the war and fight by Germany's side.
• President Pottering said: I would like to inform you that I have sent a letter to the Prime Minister and the Deputy Prime Minister to congratulate them.
• I have sent in letters on the subject.
• One of them would have sent a letter or some kind of message.
• Those who took the initiative have sent a letter to all Members to explain what exactly their intentions were in taking this action.
• In parallel, both Romania and Bulgaria's Ambassadors have sent a letter to the Council requesting that those documents should be declassified and send to all the Members.
• Peter, I sent you a letter.
• Seo In Chan's girlfriend, Han Se Gyeong, sent you a letter.
• I sent you a letter from Kabul... but the dukhs downed our aircraft.
• I sent you a letter before they knew what was wrong.
• Some lawyer sent you a letter.
• King Francis has sent you a letter.
• I would like to inform you that I have sent a letter to the First Minister and Deputy First Minister to congratulate them.
• I have sent private letters to the Prime Minister in order to get this matter cleared up politically.
• I have just sent a letter to the Division of Elections explaining that we are exhausting ourselves trying to get through this.
• I have heard that the President himself has already duly sent a letter to the City of Strasbourg.