The easiest way is to mention in your listings that you offer 'discounted shipping' for multiple orders. Ask that buyers not pay immediately, but wait for a corrected invoice.
Remember that Free Shipping really means that shipping is included in the asking price. Don't cut your own throat.
Remember that eBay will still charge you shipping FVF on the original shipping fee, so don't refund more than necessary.
If a buyer pays too quickly, you can still refund through Paypal, but watch out for the eBay shipping fees.
