Hi,
Welcome to the DocuSign Support Community!
You should reach out to the person that sent you the document. DocuSign was used to deliver the document to you and obtain your signature.
If you look at the example below, you'll see red arrows indicating who sent the document.

On the DocuSign notification email that you received, there will be similar information. That is the person you would need to contact. Ask him/her to add an attachment field to the document so that you'll be able to upload the invoice.
If you have further questions, please let me know.
Thank you!
Donna
Community Moderator