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.
Scroll down to your order and click on the "Download order details" for the information.
If you need more information, then request an Invoice from the same page.