One won't hurt you. You could look at the label on the bottle of syrup and see if they use cane sugar or high-fructose corn syrup -- cane sugar is less damaging. Don't let the "one" Italian soda be along with cookies and candy, just drink it as your one sweet thing in a couple of days and you'll be fine.
ps -- Tell the person making the soda to use a relatively light hand with the syrup. Some dump in squirt after squirt of that sweet stuff, and it isn't ever necessary for a good flavor.