If you’re dealing with a Buy It Now or fixed-price listing, you don’t have to wait and go back to the listing once you’ve purchased the item. After you confirm to eBay that you want to make a purchase, you’re taken immediately to a page with a Pay Now button.
After you click the magic Pay Now button, you’re whisked to a page where you can review your purchase. Below the Order Details section, you find a link you can click if you want to type a message to the seller.