Invoiced wrong amount

Hi there - I recently issued an invoice for an incorrect, lower amount that did not include postage.  I combined the sale of 2 listings (one with free postage and another with $11 postage) and failed to include postage.

The buyer paid that invoice - but subsequently has said they are happy to pay the correct postage.

Is there a way to issue a supplementary invoice to a buyer ?

Thanks 🙂

Make a listing for postage, and advise the  buyer of the listing number.