If they paid, I leave positive feedback. Who cares if they received it yet or not? Does that have any bearing on the type of feedback you'll leave them? Say something did go wrong. They didn't receive it, cards were damaged, etc. Then what? They send the damaged stuff back or you're forced to refund because they didn't get it. They still paid quickly, so they still deserve the positive rating. There's no reason to withhold feedback to see what the buyer is going to do when he gets them. As long as you ship properly, there's nothing to be worried about, right? If you're nervous to leave feedback right after your paid, maybe you're hiding something. Undisclosed damage, not using tracking, etc.
"He came right in and pissed on the carpet"