Santa Ponca is about 45 mins transfer, trust me, been there, lovely place, great for shops etc
We went there on the 104/5 bus a couple of times from Palmanova where we were staying.
Also caught the 102 from Santa Ponca to get to Porto d' Andraxt, that's well nice too, worth a visit.
The buses are quite cheap, but don't trust the time tables, the drivers don't use them !!!