You can find your invoices in the 'Previous Collections' section on the top right bar.
If you click on 'Previous collections', you'll see a summary of all collections that already happened, arranged by date. First, click on the drop down arrow to see the invoices for a specific day. Clicking on 'Download invoices' will open a PDF document with all the sales for a particular day and the specific Assembly you clicked on.
The 'Accounting Documents' section allows you to download more detailed accounting documents, eg. sales per quarter.