Jilembuek56
No SBB supersaver options are offered with international tickets, caused by selecting Milano as destination.
You’d need to split your tickets into a Spiez - Lugano (via Zurich) or Spiez - Domodossola part (= SBB ticket, supersaver available) and a Lugano/Domodossola - Milano part (= international or Arcobaleno Transfrontaliera ticket).