Pass-through costs

You don't actually have to prove that. You create your own invoice and if you say that your travel expenses, supplies, etc. cost €X, then that's how it is. But you can, of course, choose to send them anyway. You must include them on your invoice, because otherwise you run the risk of the same cost ending up in your accounts and in your customer's accounts, which is obviously not allowed.

But if you do send them, you must send them by email from your own mailbox instead of Dexxter. This is because you cannot add attachments in Dexxter. You can, however, download your invoice from Dexxter and send it from your mailbox. That way, you can send everything together.