You can offer two (or more) shipping choices, and check off "Free" for the first one. IIRC this can't be done if you're using your phone, you have to use an actual computer to access all of the options on the listing form.
Unfortunately, the listing form isn't clever enough to figure out how to charge the buyer only the difference between the free option and the upgraded service; it will charge the full cost for Priority.
One workaround would be to set the package weight to a lower amount so the calculator charges less, but then you'd have to remember to correct the weight when purchasing the postage label. Another workaround would be to set the shipping at a flat rate, rather than calculated, and then charge only the difference between Ground Advantage and Priority for whatever amount makes sense, such as whatever is the greatest difference between the two zoned prices (probably the Zone 8 price). You can look those up here. Use the eBay shipping calculator if you need to know how much you'll actually be paying for that Priority Mail label.
http://pe.usps.com/cpim/ftp/manuals/dmm300/notice123.pdf
