Yeah, i would be surprised if a cage company offered the option for flexwatt but didnt crimp and seal the plug wire onto the flexwatt lol. That would like offering the cage, then just sending you sheets of PVC and the glass and having you cut it yourself lol.
Also be aware that flexwatt is not going to heat the air of your cage by much. So if you keep the room under 75F, i would consider a RHP over flexwatt. I personally keep my house around 78F and the snakes are in a finished basement which is usually around 80F so i get away with flexwatt for everyone and on really cold days, i can just run my oil filled radiator in the basement.