The first step will be to sign in to your Back Market account, in order to display the summary of the order concerned:
Once you're signed in, go to your "Orders page".
Click on "My orders", you will then see the summary of your past orders,
Select the order concerned and click on: "Proof of payment". Proof of payment will be downloaded in a new tab. This document cannot take the place of a VAT invoice.
Scroll down to your order and click on the "Download order details" for the information.
To receive the invoice, you will need to request an Invoice from the seller on the same page.