Kufiege80
All regular (non-saver) public travel passes or tickets such as your STP are always available in unlimited numbers and always at the same prices. (The only exception is mid-december every couple of years, when all prices are generally adjusted to inflation).
So there’s absolutely no reason to ‘secure’ your (unlimited) STP weeks or months in advance from overseas, as opposed to (limited) flight seats or (limited) hotel rooms. Just get it at the airport SBB travel centre when arriving. You’re even avoiding unnecessary hassle with refund attempts, in case your travel plans would need to change e.g. due to medical reasons.