How to enter Combined purchases discount?

I sold 3 items to one buyer and a discount 0f $10.00 was agreed via messages. I thought I could combine the purchases and enter the 10.00 discount into the invoice, but I have entered the (-10.00) discount and it won't go through. Can you help please? Item numbers are 331471629610, 331471661468, 231472713486.
Regards,
Rose
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Then only way you can discount is by reducing postage as far as I know. 

 

So if for the total items the postage would be $20 and you want to discount by $10 - you enter $10.00  in the postage area to over ride the calculated amount and send the invoice.

 

DO NOT click recalculate as this will revert the invoice to the original amount.