Creating an invoice?

A buyer has purchased 4 items from me and i want to send an invoice so that i can combine (& reduce) the postage costs but i cannot find any options for me to do this?
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Go to the right of one of the items purchased,  and from More Actions drop down menu select Send Payment details. All items will appear on the one invoice.    Amend the postage,  but do not click on reset.  Check the invoice is now correct,  and send to the buyer for payment.