How do I send a copy from an invoice?

Go to that sales invoice by clicking on the number of your sales invoice.

You can then download it (top right of the document) or click on "send" again. These seem like questions that you might be better off asking Dexxter via the support button.

You will also get a quicker answer than if you ask here, or at least normally :)

When I click on the invoice nr., I am taken back to the menu where I can edit the invoice, but I cannot see my invoice anywhere.

Income>Sales invoices>Actions column> ... Download