I can't use the invoice as a reference to my expence, as the invoice does not reflect what was paid in case I got CREDITS.
If I get $100 credits, the invoice still states the total amount of money I "should pay".
I can only reffer to the email sent to me - and that does not have enough properties either.
Thus: I can't really use DO, as I have no way of getting my money back from this expense. :-)