What your vet gave your snake was just ivermectin. If the dosage isn't correct, it can cause some problems, such as neurologic disorders. The Provent-A-Mite works just fine. The mites have to get off of the snake to lay eggs, and when they do, they die.

PAM the cage, even Reptile Relief the snake if you must also, then be done with it.

Those shots HURT, ivermectin is THICK almost like penicillin, and it has to go through the snake's entire system to kill the mites. The mites have to BITE the snake and get ivermectin IN their bodies before they will die. Also, reactions to ivermectin aren't all that rare either, and mites these days are getting more and more resistant to different treatments.