The contract is concluded between you and the client directly. Please make sure it is legally solid and, if you're unsure, prefer a Dienstvertrag to a Werkvertrag.
If the client asks you for a contract, feel free to use our template for a Dienstvertrag.
You issue your invoices directly to the client. Please forward each invoice to us as soon as you sent it to the client, so we can send you an invoice back for our commission.