Once you've dropped off a sold book at the Post Office or in a self-service bin, you can mark the book as shipped! Go to your account in our app, select "My Sold Books" in the account tab and then the individual order. On the next page, you'll see the option to mark the book as shipped. This will allow the buyer to see that you've dropped off the book.
For more general info on shipping on Pango, see this article.