Credit Note

Hi, could you please add the ability to add credit invoices / credit notes ? 

How does that exactly work? Can you elaborate, please?

If a user wants to send purchased goods back he will also need a credit.

Thus I need to be able to add negative values in the invoice or create another of type "credit"

this would be good. I run markets and if a market gets cancelled I need to be able to credit my vendors for the next market. and make a note of it.