Pour 1/3 to 1/2 cup of the syrup into each jar so that the jar is about two-thirds full of liquid. Use the bubble remover or chopstick to gently jostle the peaches to make sure the liquid sinks to the bottom. Top off with about 1/2 cup grappa, leaving 1/2 inch headspace. Screw the lids on tightly and process for 25 minutes in a boiling-water bath. The peaches will keep, properly sealed, for 6 months; after that they begin to lose their color and texture. Once opened, store in the refrigerator for up to 1 month. If a jar fails to seal properly, refrigerate it and enjoy those peaches first.