Is “postchoice” a well-used word?

Personally I think postchoice is relatively new "jargon", so yes - it's still "current" (so much so that it's still up for grabs whether to hyphenate it or not).

It's normally used in technical/academic contexts - most commonly, in respect of how, in decision-making, we evaluate relevant factors differently pre- and post-choice (things favouring the decision we actually made are often rated more highly afterwards, thus justifying our choice).

With issues of postchoice satisfaction (which concern marketing people just as much as therapists and psychologists) the word is used as an adjective, modifying the noun "satisfaction".

I don't understand OP's “postchoice” used as a postposition to “options”. In his quoted example, the word postchoice adverbially modifies the verb [phrase] "re-evaluate [the options]".