I used to leave feedback on receipt of payment but tend to leave it after a few days now. If they haven't given feedback but probably do have the item, I'll leave feedback as sort of a reminder... they might have forgotten/chosen not to leave it but if they notice I did, it might spur them on to say "thanks" or whatever their feedback might be. The reason for the wait is incase they give an unjustified neg, I haven't jumped the gun.