No offense, but I don't think any of you guys have a clue..
It isn't just YOU who is sending a letter to him, via venue isn't some secret thing that no one knows about. He gets hundreds and hundreds of letters each concert, and there's just no time to open every single one, read the letter that each person sent, sign their crap, stuff it back in the envelope, and send it back. He's busy preparing for the concert, doing soundchecks, doing meet and greets, and warming up his vocals.
For example, on his website a few weeks ago, it asked fans to submit questions to him - they received over 20,000 responses. Granted, that is different than people sending autograph requests to him via venue - but it is an overwhelming response due to interest in Paul.
He doesn't have time to sift through the thousands of letters that he receives during the duration of his tour. Give the guy a break, he's been doing this for 50 years and has probably signed hundreds of thousands of autographs.