In the myeBay view you can send a revised invoice if the buyer has gone through checkout but has not made payment.

Next to the item, under the Print Postage Label blue button, select more actions, then about 6 or 7th choice down the list that appears there will be send invoice.

When the new invoice is generated it should have both items listed.  Adjust your post price, do not hit recalcuate as this fuction just clears your adjustments, just send (or view and then send from view screen).