Since refunds aren't issued until requested by the buyer, communication is key. So, if you need additional time, you can select the Report Shipping Delay by scrolling to the bottom of the Order Detail screen (see image below). This will generate a pre-populated message to the buyer (feel free to edit this message to suit the situation) informing her that you need more time.
In most cases, buyers are fine with the delay so long as they are aware of the situation. However, if the purchase is time sensitive, they may still choose to request a refund.