I think it started when the carrier had something to hand in through the door and, seeing Charlie babygated upstairs, asked to meet and pet him and give him a treat. Then, when we were out walking, he went out of his way to come over and pet Charlie, say nice things about him, and let him rummage for treats. Then at some point, whenever there was a package to be left by the door, he'd add a treat on top:
And now... oftentimes after the mailman's come and gone, look what's been left in the middle of the welcome mat:
What a great mail carrier!