Flat Tire - Valve Stem fell through into inside of tire. Can I replace just the Stem or does it have to be whole TPMS?

Hello! It's my first time posting - I've tried to look online but haven't had
much luck and was hoping someone could offer insight. I have a 2009 Jeep
Grand Cherokee Lardeo and bought new tires last year (there were 55,000
miles on my Jeep). The day after my tires were replaced, the cap popped
off when I was on the highway and I got an instant flat. I took my Jeep back
to the store that replaced the tires and they put a whole new one on and
replaced the "TPMS Grommet Kit that had failed" for free (this language is
directly off of the invoice). The other day I was driving and the low pressure
light came on, I checked the tire and heard air blasting out. I tried to re-
position the valve a little and it helped temporarily - enough for me to drive
home. The next morning I went out to check on it again and the tire was
flat. The whole valve dropped into the inside of the tire. Sorry this is such a
long explanation - anyway, I was wondering if with this year and model
whether or not the valve comes off of the sensor and can be replaced that
way or if the entire TPMS is completely attached and would need to be
replaced. There is obviously a big difference in cost ($6 or so vs. $40+). I'm
going to take it back to the store today (I have a full size spare I've been
driving on), but I don't want them to scam me saying the whole system
needs to be replaced if the stem valve can just get removed and replaced
for much cheaper. Thanks so much for your help!