Runn doesn't generate or track invoices and payments, however there are a couple of ways our customers manage this:
You could use the 'project milestone' feature to add invoicing milestones & 'milestone note' to update when the invoice has been issued and paid
You could add the information to 'project notes' which is right at the bottom of the 'project details' view
You could export the hours from your projects as a csv from the project dashboard and import to your invoice tool
We don't currently have invoicing as a feature in our roadmap, but if you are interested in this or any other features, let us know by sending an email with details of the request to help@runn.io