For every purchase you make on Daan.dev, you'll receive an invoice, which can be retrieved in multiple ways.
From the Order Confirmation
When you place an order on Daan.dev, you'll receive an order confirmation. This order confirmation contains a link, called View Invoice.
From your Account Area
When logged in, go to Your Orders:
In the table of orders, you'll find a column, called Invoice:
Click on View Invoice and your invoice will open in a new window.