Hello PamN
When buying the Swiss Travel Pass Flex you could actually define the time period in which you want to use the travel days; each travel day is to be activated latest before use. That’s the flexibility the STP Flex offers, yet it is very costly.
I, personally, would not purchase a Swiss Travel Pass Flex, especially if I needed it for less than 15 travel days; it is simply too costly.
You are better off purchasing the Swiss Half Fare Card for 1 month, plus Saver Day Passes, or individual tickets, especially if you are unsure about your travel plans. Simply do the maths and double-check whether you would indeed be spending the daily rate of a Swiss Travel Pass Flex day compared when purchasing individual tickets.
Kind regards,
Roland