If I ship (get something out same day) or next day, the lightning fast shipping almost always gets me an excited 'positive feedback'. If I slack on a speedy shipment, wait a few days, it affects my feedback. get it shipped as soon as possible. Get it to them in 5 days or less and watch how happy it makes them.
About the autoresponder -- I think (I'm not sure) that it comes with the Selling Manager Pro.
I'm not sure if it is available without it. Try asking, eBay's Live help.