For the advance purchase fares to Plymouth, you need to be travelling from beyond St. Austell, they are not available for St. Austell to Plymouth on FGW▸ but are available on Cross Country trains.
Alittle secret, some people who travel from Lostwithiel/Bodmin buy a leisure advance from Truro as it works out cheaper.
I did try that but on that day, there were revenue standards inspectors at St. Austell so I had to buy a standard day return. So now I use a monthly season ticket which gives me the flexibility I need.