PAM is expensive because it works, products found at pet smart do not, they will not take care of the eggs only the adult stage. Just like you can't cure a RI with a cheap Vicks treatment either.
There are home brew but if you do not know what you are doing you could simply kill the animal so I won't recommend those.
You can also get a can of RID and use it the same way as PAM
Personnally if it were me the animal would be shipped back, and I would get my money back. Between the RI and the regurgitation you have experienced recently with other animals do you really want deal with that too, remember mites can carry diseases, and what if mites are not the only problem.
And for future reference if you acquire animal you should ALWAYS pre-treat for mites while in QT