As a seller, I provide feedback as soon as payment is received, too. I thank them for their fast payment, and tell them what day the package shipped.

As for buyer feedback, there used to be a convenient link on "My EBay" to leave feedback. The way eBay has jacked up their menus, it is no longer convenient to find all currently pending transactions needing feedback with a single click. Heck, I even have trouble finding all the transactions where I have won auctions - the "My EBay" feature no longer has a number after each type of transaction to make life easy. I just attribute the lack of buyer feedback to the inconvenient way eBay's menus are set up.