Are you on a desktop or a mobile device? With the desktop, you might be able to manually 'mark as payment sent' or 'hide' it but there are limitations to the app that don't allow you to do the same. It's kind of annoying, I know. For sellers too.
Mark as Payment sent used to be in the drop-down menu you'd find in Purchase History at the item itself. I don't know if they moved it. I have no unpaid items myself so I can't check right now to tell you.
